The Seneca Indians called the area Sehgahunda, meaning the "Vale of Three Falls," forming several villages within what are now the park boundaries. Many of the trails in the park today are based on original Seneca trails, which were used for river access.

In the middle of the 19th century, when the Erie Railroad needed to bridge the gorge, they built the world's tallest wooden bridge right above the Upper Falls. The view from above was so spectacular that trains would stop and let passengers off to take in the sight. On one such voyage was Buffalo industrialist William Pryor Letchworth; he so enjoyed the view that he bought over a thousand acres of gorge land and built himself a summer home. He called the estate Glen Iris after the rainbow that often formed in the mist of the Middle Falls. As his death approached, the elderly Letchworth decided to protect the gorge from hydro-power or other commercial development by deeding it to the State for use as a park in 1906.

Despite its nickname, the valley carved by the Genesee River doesn't really look like the Grand Canyon; visitors who've seen the real thing won't be very impressed by the comparison. For starters, it's much smaller in scale. At its widest point, the Grand Canyon is 18 miles (29 km) wide; Letchworth Park would just fit inside there—lengthwise! And the Grand Canyon's mile-high depth dwarfs the 300- to 500-foot-plus (100- to 150-meter-plus) walls of the Letchworth gorge. On the other hand, Letchworth has far more vegetation in and around the gorge, and it's fair to say there are few other gorges this side of the Mississippi that approach Letchworth in either size or beauty.

Geologically, the Letchworth gorge is very young, carved when the Genesee was forced to find a new route northward after the last ice age. The preglacial river that flowed through current-day Portageville was a tributary of the preglacial Genesee River, and it took a more easterly course through Nunda. It then joined the main branch of the preglacial Genesee, which flowed northwest to Mount Morris, then north to Rochester. When the two branches of the river were blocked by glacial debris at Dansville and Nunda, the east branch rerouted to the south, becoming part of the Susequehanna watershed, while the west branch was forced into a more direct route between Portageville and Mount Morris.

The new route led the river over the Allegheny Plateau, dropping into the preglacial river valley at Mount Morris. Over the next ten millennia, the cascade carved its way backward, much as the Niagara River did during the same period, eventually forming a series of three waterfalls just north of modern Portageville... and a spectacular gorge in its wake. The course the gorge takes is a bit unusual; it twists and turns rather than taking a straight path; since the river once flowed in a meandering route over a broad plain, the falls cut their way backward along the original course of the river.

The three cascades of the upper Genesee should not be confused with three similar waterfalls farther downstream (to the north) in Rochester. Both locations have a Lower, Middle, and Upper (or High) Falls, and both sets were formed during the same period, but they are more than 50 miles apart.

Unlike the Grand Canyon of the West, Letchworth's gorge walls are covered in trees, making for spectacular sightseeing in the autumn, as the leaves change color. Birdwatchers love the rim of the gorge for spotting hawks, eagles, and vultures soaring overhead.

Like much of the northeastern U.S., the park lies in the humid continental climate zone, with four seasons. Normally summer highs are around 70-80°F (20-27°C), whereas the winter temperatures are generally around freezing or a bit below. There aren't radical differences between the monthly precipitation throughout the year. In the winter you can expect it in the form of snow, in the case of lake-effect snow, quite a lot of it.

The most convenient expressway to Letchworth is I-390, which runs south from the Rochester airport and passes near Mount Morris as it travels through the preglacial Genesee Valley. If you're going to the north end of the park, take the Mount Morris exit (Exit 7); if you're going to the south end, you can take Exit 7 and drive through or around the park, or take Exit 4 or 5 in Dansville and follow State Route 436 west to Portageville. The major east-west expressway routes are the New York State Thruway (I-90), about 20 miles (30 km) north of Mount Morris, and I-86 (the Southern Tier Expressway), about the same distance south of Portageville; both connect up with I-390 or any number of state routes that are more direct.

The park has six entrances, but choose wisely, as there's no way to cross the river (by car, at least) once you're in the park. All of the amenities, including the main park road, are on the west side of the river; the east side has a number of trails and a couple of campgrounds, but is largely wild and undeveloped. On the other hand, only the west side carries an admission fee.

On the east side, the 42.579877-78.0288191 Parade Grounds Entrance is to the south near Portageville, off New York State Route 436; it's closed in winter, but the rest of the year leads to the only cabins on the east side. Its trails provide east-side views of the falls and gorge that many visitors never see. There's also a 42.709732-77.901832 Mount Morris Dam Entrance on the north end, accessible from New York State Route 408 south of Mount Morris, which only provides access to the dam's visitors center (along with a nice gorge overlook).

Most visitors will use a west-side entrance. The 42.740525-77.8903723 Main Entrance is all the way at the top of the park; take New York State Route 36 West (northwest) from Mount Morris and across the river (or eastbound from Leicester, a couple miles to the north) and turn onto the Park Road. There's also access to the Park Road from the 42.694314-77.9582264 Perry Entrance and 42.601061-78.0372235 Castile Entrance, each farther south. Castile is the southernmost entrance open in the winter; the entrance at the southern tip, the 42.571718-78.0517826 Portageville Entrance is closed in winter. The Portageville entrance is on New York State Routes 19A and 436; the former comes in from the south, through Portageville, while the latter runs east-west and connects to Nunda and Dansville. The Portageville and Castile entrances provide the best access to the falls, the most popular gorge views, the Glen Iris Inn, and the highest concentration of lodges, cabins, and concessions.

To get from one side of the park to the other, you can cross at the north end on Route 36 or at the south end on Route 436, but either way you'll have to re-enter the park at another entrance.

Infrequent and limited bus service is provided by RTS. This includes limited Saturday seasonal service on the designated Letchworth route^{[dead link]} into the park, as well as infrequent service to Mount Morris, where you can then walk into the park. Trailways and FlixBus also serve Mount Morris. Plan ahead carefully as service is very limited.

Part of the park's trail system is connected to the Genesee Valley Greenway, a multi-use rail trail that extends 90 miles (140 km) from the Erie Canal in Rochester all the way down to Cuba in the Southern Tier, along I-86. The trail crosses the river on Route 36 in Mount Morris, right at the north end of the park; one of the park's trails picks up just inside the park entrance. From there, the Greenway heads southeast through Mount Morris and Sonyea, then southwest through Tuscarora, Nunda, and Oakland before entering the park north of Portageville; from there you can access the east-side facilities and trails near the Parade Grounds entrance.

Both Buffalo and Rochester have mid-sized airports with plenty of flights daily from all the major domestic hubs and fully staffed rental car agencies. There are a number of smaller airstrips in the surrounding countryside, but then you're stuck without a car to take you the rest of the way.

Located near the south end of the park, these are Letchworth's primary draws. Though the views of the falls and gorge are great year-round, the place really shines in autumn; it's one of the best sites for leaf-peeping in the entire Northeast.

The 42.578515-78.0488991 Upper Falls, whose drop totals 71 feet (a 50-foot main drop and a few rapids just below), cascade over a tight horseshoe bend, making them a little hard to see. But the Portage Bridge just above the crest makes for a dramatic backdrop. The 42.583085-78.0426982 Middle Falls are within easy walking distance of the Upper via a paved trail, and they are both the tallest (107 ft) and the most picturesque. The rainbows of the Middle Falls captured the imagination of both the Seneca Indians (who believed the beauty of the falls made the sun pause at midday) and William Pryor Letchworth, who named his estate Glen Iris after the rainbow-filled valley of the Middle Falls.

The 42.585369-78.0212493 Lower Falls are a bit further downstream, and they have the shortest drop (70 feet). You've got to climb down a substantial set of stairs to get to any decent viewing location, but it's worth it, as the Lower Falls' face is the most interesting of the three. There's a stone footbridge just below the Lower Falls, the only (legal) way to walk across the river within the park.

While the three main waterfalls capture most of the attention—rightfully so—there are a number of other waterfalls in the park, each representing a tributary of the Genesee that now has to drop all the way down into the carved gorge. That makes for some very tall falls, though they tend to be fairly small in width and volume. Some even dry up seasonally. But with total drops approaching 500 feet, the heights some of these falls reach can rival the volume of the Genesee Falls for sheer spectacle.

Once you're done gazing at the falls, there are a few more attractions to check out.

• 42.5842-78.04334 Letchworth State Park Visitors Center, ☏ +1 585 493-3600. There are concession buildings with restrooms and guides in several places in the park, but the main visitors center is along the park road between the Middle and Lower Falls, not far from Inspiration Point, north of the Glen Iris area. 
• 42.5851-78.04385 William Pryor Letchworth Museum (Glen Iris grounds), ☏ +1 585 493-2760. May-Oct: daily 10AM-5PM. In 1898, Mr. Letchworth himself erected a "Genesee Valley Museum" building on his estate grounds. He used it to exhibit his collection of Native American artifacts and geological specimens—including a mastodon skull! Though the collection has been expanded and moved to a bigger building since that time, it still maintains his original focus on the human and geological history of the Genesee Valley. The museum also memorializes Letchworth himself, housing his personal library and other documents and artifacts from his life. And the original mastodon skull still astounds visitors today. Recommended donation: $1/adult, $0.50/child, $3/family. 
• Genesee Arch Bridge - Active Norfolk Southern railroad bridge carrying freight traffic through the park with defect detectors usually giving a readout either before or after the train crosses the bridge.
• 42.58786-78.038616 Council Grounds (just north of Glen Iris). Letchworth took a keen interest in the Native American history of the Genesee Valley, and when he heard that the old 1765 Seneca Council House in nearby Caneadea was in danger, he had the whole thing taken apart and reassembled on his estate grounds. Later, at the request of relatives, he relocated the grave of Mary Jemison, the "White Woman of the Genesee", from the endangered Buffalo Creek Reservation near Buffalo to the new Council Grounds at Glen Iris. A cabin, built by Jemison for one of her daughters and formerly located on her property downstream at the Gardeau Flats, was also relocated. Today, the two buildings and the gravesite (marked by an iconic statue) continue to be carefully preserved. Free with park admission. 
• 42.592739-78.0347117 Humphrey Nature Center, ☏ +1 585 493-3680. Daily, 10AM-5PM. Open year-round, the Humphrey Nature Center has exhibits, many of them interactive, illustrating the geology and biology of the park. Free with park admission. (updated Jun 2018)

• 42.733333-77.9069448 Mount Morris Dam, 6103 Visitor Center Rd, Mt Morris (Access from Route 408, south of Mt Morris), ☏ +1 585 658-4790. Visitors Center open Apr-Oct, daily 10AM-4:30PM; tours Apr-Oct, daily 2PM plus F-Su 11AM; grounds open all year, daily 6AM-11PM. Communities along the Genesee River suffered under periodic flooding throughout the 19th and early 20th centuries. The Great Flood of 1865, for instance, devastated Rochester, and a repeat was seen as increasingly inevitable. Around 1950, the Army Corps of Engineers constructed the massive Mount Morris Dam, using the deep valley to the south as a reservoir, allowing the regulation of northward flow. At 230 feet (70 m) high and over 1000 feet (300 m) long, the dam is an impressive sight. A Visitors Center was created in 1999 on the east side of the river; it contains exhibits on the Dam, the U.S. Army Corps of Engineers, and earth science. Free walking tours that go inside the dam structure depart once or twice daily. The grounds have picnic areas, playgrounds, and trails. Free.  
  • The dam can also be viewed from the west side of the river, just off the main Park Road; there's no access to the dam or to the main Visitors Center from there, but it's close to the main park entrance, so there's a parking lot and concession stand with souvenirs and food.

• 42.5845-78.04291 Glen Iris Inn, 7 Letchworth State Park, Castile (Park Road between Castile and Portageville entrances, at the Middle Falls), ☏ +1 585 493-2622. "Glen Iris" is the name William Pryor Letchworth gave to his country estate, where he entertained friends and business associates above the Middle Falls of the Genesee. Today, guests can relax in the very same building, or in one of several out-parcels. The 12 standard rooms are a bit small, with either one double bed or two twin beds, but each has a private bath. The four suites are larger, each with a queen bed, but almost twice as expensive; the largest has a private porch and a whirlpool tub. The adjacent Pinewood Lodge offers seven efficiency units, each with a kitchenette, queen bed, and single-size sleeper sofa. There are also three smaller houses not too far away, offering multiple bedrooms and full kitchen facilities for larger families and groups of up to eight people. WiFi is available but a little slow. $100-400.  

• Letchworth Campgrounds, ☏ +1 800 456-2267. The park camping season lasts from early May into mid-October. Prices range from $21 to $27 per night per site. ($5 daily surcharge for out-of-state visitors.) Reservations are made through Reserve America. 
  • 42.697274-77.9336452 Highbanks Camping Area. The tent and RV camping area is located near the Perry Entrance. Pets (max 2) allowed in 100, 200, and 700 loops. 30 and 50 amp electric sites. 
  • Cabin Areas. Cabins are available for an extra month on either side versus campsites. Prices range from $30 to $115 per night, with a $6.25 daily surcharge for out-of-staters. All five areas offer cabins with electricity, a stove, and refrigerator. Most are heated, other amenities vary. No pets allowed. Cabin areas D and E are only accessible via the Parade Grounds entrance (eastern side). 
    • 42.5941-78.00913 Cabin Area A. Cold running water (during peak season). 
    • 42.5898-78.01434 Cabin Area B. Cold running water (during peak season) 
    • 42.71325-77.933195 Cabin Area C. The only cabin area in the northern part of the park, most open year-round 
    • 42.5832-78.0326 Cabin Area D. Cabins 3, 8 unheated 
    • 42.5973-78.0027 Cabin Area E. Unheated, no shower facilities 

Once you have arrived, check in at one of two places.

• 42.59059-78.026981 Visitor Center. Cabin areas A, B, D, E (updated Jul 2016)
• 42.69311-77.955192 Contact Station. Campground, Cabin area C (updated Jul 2016)

The 584-mile main leg of the Finger Lakes Trail runs through the park for approximately 25 miles along the much less visited eastern side of the gorge. There are two lean-tos along this trail, however a permit must be obtained from the New York State Parks Department in order to camp overnight.