The state highway was designated when the MD 32 bypass of Sykesville opened in 1963.

The state highway heads north as two-lane undivided West Friendship Road along the edge of Hugg-Thomas Wildlife Management Area into the narrow valley of the Patapsco River.

MD 851 traverses the river and crosses CSX's Old Main Line Subdivision railroad line at-grade as the highway enters the town of Sykesville in Carroll County, where the highway's name changes to Main Street.

[6] MD 851 was relocated north of Third Avenue in 2006 to create a perpendicular intersection with MD 32, which was also relocated as part of the project, and to provide a better connection from both state highways to Springfield Hospital Center.

These highways are sections of old alignment of MD 32 between West Friendship and Eldersburg.