Known as Hunting Creek Road, the state highway runs 4.43 miles (7.13 km) from Lowery Road near Huntingtown north to MD 524 in Huntingtown.

MD 521 begins at the intersection of Lowery Road and Soper Road southwest of Huntingtown, near the confluence of Hunting Creek and the Patuxent River.

The state highway crosses Little Lyons Creek and heads north as a two-lane undivided road through a mix of farmland and forest, where the highway intersects the other end of Lowery Road.

At the intersection with Huntingtown Road, MD 521 turns east into the center of Huntingtown, where the highway reaches its northern terminus at MD 524 (Old Town Road).

[3][4] Aside from paving and minor improvements, the state highway has not changed since the 1930s.