The route winds north through Bedford as a two-lane road past Pea Pond before reaching the southern end of the Cross River Reservoir.

Leaving Cross River, NY 121 continues north through Lewisboro, returning to the dense woodlands.

After passing west of Waccabuc, the route reaches a junction with the terminus of NY 138 just before crossing the town line from Lewisboro into North Salem.

[3] Now in the town of Southeast, NY 121 runs alongside the Vail's Grove Golf Course and soon into exit 69, a pair of ramps from I-84.

Just north of the I-84 interchange, NY 121 terminates at an intersection with US 6 and US 202 in front of the East Branch Reservoir.

NY 121 was designated in the 1930 renumbering of state highways in New York to its current alignment.