Known as Old Frederick Road, the state highway runs 7.57 miles (12.18 km) from MD 32 near West Friendship east to U.S. Route 29 (US 29) in Ellicott City.

MD 99 parallels the north side of Interstate 70 (I-70) through a rural and suburban area in northeastern Howard County.

[1][2] East of Woodstock Road, MD 99 crosses Davis Branch and enters a suburban area on the north side of Ellicott City.

The roadway continues east as county-maintained Rogers Avenue, which curves south toward the historic downtown of Ellicott City.

The highway, which was mostly built along the height of land between the Patapsco and Patuxent watersheds, was replaced by the more southern Baltimore and Fredericktown Turnpike, which is now MD 144, around 1812.

[3] In the 20th century, the portion of Old Frederick Road east of West Friendship became part of two state highways.

The original MD 100, as opposed to the modern freeway that connects Ellicott City and Glen Burnie, was assigned to Old Frederick Road east of St. Johns Lane.

[9][10] Construction began shortly after 1930 on the gap between St. Johns Lane and Wheelwrights Gate; MD 100 was completed when that section of Old Frederick Road was paved in concrete by 1933.

[11] Rogers Avenue had been reconstructed and widened to 24 feet (7.3 m) starting in 1953, shortly before it became part of MD 99.