The highway's northern end was moved to its present terminus just north of I-270 when Stringtown Road was constructed in the mid-2000s.

MD 121 heads north as a two-lane undivided road that crosses the Tenmile Creek branch of Little Seneca Lake.

The highway passes through the hamlet of Burdette and intersects Old Baltimore Road, which provides access to Black Hill Regional Park to the east.

[1][2] The first segment of MD 121 was constructed as a macadam road from Clarksburg to north of Tenmile Creek by 1910.

[8][9] Washington National Pike (now I-270) was constructed through Clarksburg and the freeway's interchange with MD 121 was built between 1952 and 1954.

[12][13] Between 1984 and 1987, MD 121 was relocated at its southern end at Boyds due to the construction of Little Seneca Lake.

[22] That same year, Schaeffer Road became MD 119 again for a short time until it was turned over to Montgomery County in 1958.

[22][23] The 1.00-mile (1.61 km) segment of MD 121 along White Ground Road was transferred to county maintenance in 1999 as part of a highway swap to designate Great Seneca Highway as MD 119 (which ironically, was the original number for White Ground Road).

The highway begins at a cul-de-sac adjacent to MD 121's northern terminus just north of its I-270 interchange.

Just north of the cul-de-sac, MD 121A meets the western end of Gateway Center Drive, which links the route with Stringtown Road.