It begins at an intersection with U.S. Route 302 (US 302) and SR 35 in Raymond and ends in Auburn.

It is signed as a north-south highway, which accurately reflects its alignment from Raymond to Oxford, but from there it runs almost due east (and slightly south) to its terminus in Auburn.

It continues north along the west side of Pleasant Lake and Thompson Lake before entering Oxford, where it turns to the east to meet SR 26 just south of the Oxford County Regional Airport.

SR 121 briefly joins SR 26 in a wrong-way concurrency before splitting off southeast towards Mechanic Falls (continually signed as "north").

[2] However, per Maine Department of Transportation documentation, SR 121 continues north on Washington Street (the northbound direction for US 202, SR 4, SR 11, and SR 100), meets the southbound lanes at Minot Avenue, and ends at the intersection of Court Street, Union Street, and Minot Avenue.