SR 121 begins at a partial cloverleaf interchange with I-5 and Maytown Road near the community of the same name, located east of Littlerock in southern Thurston County.

[3][4] The highway then turns north onto Tilley Road and crosses over the railroad, traveling around Deep Lake and reaching Millersylvania State Park.

The highway turns west onto 93rd Avenue Southeast near the runway of Olympia Regional Airport and travels through an industrial area and past a truck stop before reaching its northern terminus an interchange with I-5 in southern Tumwater.

[4][6] The two-lane highway is 7.67 miles (12.34 km) long[1] and travels through predominantly rural areas, serving primarily as a connector to Millersylvania State Park and Olympia Regional Airport.

[12][13] Maytown Road remained under Thurston County's maintenance, which included a major rebuilding project in 1954 following the completion of a freeway interchange with U.S. Route 99 (US 99), later part of I-5.