In Pennsylvania, the highway travels through the length of the Delaware Water Gap National Recreation Area, along the southern part of the Poconos in Monroe and Carbon counties through Jim Thorpe and along parts of the defunct historic Lehigh Canal and Lehigh Valley Railroad then over the divide near Nesquehoning into the Schuylkill Valley along Panther Creek.

For part of its route in New York, US 209 runs alongside the defunct Delaware and Hudson Canal, which ran from Port Jervis to Kingston,[4] in each case, following the old land road connections connecting the anthracite coal fields of Northeastern Pennsylvania with the industries and heating customers in New York City.

[9][10] Although signed as a north–south route in both states for its entire length, US 209 actually runs closer to east–west along its southern sections in Pennsylvania, only gently trending northward.

Beyond that, the generally straight route starts to curve a little more frequently into Pottsville, after which it follows the upper Schuylkill River as it heads into lightly populated areas in the Coal Region such as Port Carbon, Cumbola, New Philadelphia, and Middleport on its way to Tamaqua.

At the south end of the town, it crosses the river and resumes its north-trending eastward course, which brings it to an interchange at I-476 (Pennsylvania Turnpike Northeast Extension).

Once again, there are no major settlements along US 209 as it heads through isolated valleys, this time with more agricultural use evident, into Monroe County and eventually to its absorption into PA 33.

after several miles and taking its more firmly northeast bearing to eventually run along the Delaware River shortly after entering Pike County, its last in the state.

The two highways eventually start to run alongside I-84, and development picks up as they approach Matamoras, the easternmost town in the state.

After leaving the city, it enters the valley between the Shawangunk Ridge and the Catskill Plateau to its west, following the Neversink River until crossing it just prior to the hamlet of Cuddebackville.

In Wurtsboro, shortly after the NY 17 (future I-86) interchange, the route reaches a traffic light at Sullivan Street.

The valley begins to widen as another road, NY 213, joins for a mile before leaving at a blinker in downtown Stone Ridge.

To the north, the road eventually becomes a two-lane expressway, then a four-lane freeway just short of NY 28 just west of Kingston, just inside the Catskill Park.

This corridor, first used for long-distance transport by the Old Mine Road in colonial times and then the historic Delaware and Hudson Canal in the early 19th century, keeps it in the scenic valley between the Catskill Plateau and the Shawangunk Ridge.

Except for the freeway section at the northern end—the remnant of a much more ambitious plan to make the entire roadway one[12]—US 209 remains a two-lane rural road for much of its length in the state.

[13] On May 31, 1911, as part of the Sproul Road Bill,[14] the highway was assigned several Legislative Route numbers, each corresponding to a specific section of what would become US 209.

At what is now the interchange between US 209 and US 209 Business, US 209 broke from its previous alignment and continued east to a newly built freeway (modern PA 33).

[21] In 1995, commercial vehicles began running in the area again, as long as they pay fee at two toll booths, one in Bushkill and one just south of downtown Milford.

[22] In 2011, after rains from Hurricane Irene in August and Tropical Storm Lee in September, the grounds under the highway were saturated and after a landslide occurred on October 21, the National Park Service closed US 209 between PA 739 in Dingmans Ferry and the North Contact Station just south of the Milford–Montague Toll Bridge.

[24] Delaware Township has asked the Pennsylvania Department of Transportation (PennDOT) to keep PA 739 near US 209 clear of snow, due to the importance of the intersection.

[26] There are plans to construct two roundabouts along US 209 in Brodheadsville - one at Pleasant Valley Lane and the other at PA 115 - in order to alleviate traffic congestion.

[28] The junction where US 209 intersects with PA 402 in the hamlet of Marshalls Creek within Middle Smithfield Township was plagued with traffic for several decades.

[31] The third stage of construction of the bypass was originally projected to be completed in late 2012, but opened on June 11, 2012 with a ribbon cutting ceremony.

[33] Before the designation of the New York highway system, what is now US 209 was part of the Gap Way, which ran from the Pennsylvania border at Port Jervis to Kingston.

A year later, AASHO modified the definition of US 6, placing the route along a new alignment farther south in the state.

[7] The road changed designations for the final time in April 1935, rejoining the U.S. Highway System and becoming part of an extended US 209.

Two such reroutings were in the vicinity of the hamlets of Spring Glen and Napanoch, where US 209 was initially routed on Phillipsport Road and Main Street, respectively.

[44] Due to increasing suburbanization and a rapidly increasing population in the Stroudsburg area, I-80 is to be widened to three lanes in each direction from its current two between I-380 (exit 293) in Pocono Pines and the Delaware Water Gap Bridge (New Jersey state line), and part of this project includes the entirety of US 209’s concurrency with I-80.

This section of road was built in the 1920s and is one of the oldest stretches of the US highway in the US, starting out as a simple bypass of Stroudsburg for US 209 before becoming part of I-80.

It has one of the highest accident rates in Pennsylvania due to major issues such as most entrances not having acceleration lanes, multiple overpasses that are structurally deficient, and shoulders that are as narrow as one tenth the required length for Interstate highways.

was extended southward to its current southern terminus in 1964 when US 209 was relocated onto a bypass paralleling its former alignment between Stroudsburg and Sciota.