It begins in the city of Rye at an intersection with U.S. Route 1 (US 1) and runs for about 18 miles (29 km) north to the hamlet of Millwood, where it ends at a junction with NY 100.

The commercial surroundings give way to residential neighborhoods after three blocks as NY 120 winds northward through the northern part of Rye to the adjacent village/town of Harrison.

[3] In Harrison, NY 120 passes several mansions on both sides of the highway as it gradually turns northwestward and approaches I-287 (the Cross Westchester Expressway).

[6] Another stretch of homes north of Purchase leads to Westchester County Airport, situated between NY 120 and the New York–Connecticut state line about 1 mile (1.6 km) to the east.

Here, NY 120 begins to run alongside I-684 as a four-lane undivided expressway and both roads pass by Rye Lake, an offshoot of the Kensico Reservoir.

From here, I-684 continues north into Connecticut while NY 120 takes on the King Street name and turns northwest to straddle the state line for roughly 1 mile (1.6 km).

The route passes over I-684 and the northern tip of Rye Lake and serves an industrial complex in the Connecticut town of Greenwich before the state line turns 90 degrees to the northeast.

[3] North of the reservoir, NY 120 crosses the northernmost part of the Bronx River and traverses the northeastern corner of the town of Mount Pleasant, where the woods give way to a handful of residential neighborhoods.

The transition in surroundings continues into the adjacent town of New Castle, home to housing tracts that become less isolated as the road veers westward toward the hamlet of Chappaqua.

NY 120 turns northward here, crossing over the Saw Mill River and leaving Chappaqua for less densely populated areas of the town of New Castle.

[3] Here, the two routes cross the right-of-way of the former Putnam Division of the New York Central Railroad and pass by the site of Millwood's station, which was demolished in May 2012 after years of neglect.