State Highway 51 serves as a north–south route between Saint Paul, Falcon Heights, Roseville, and Arden Hills in Ramsey County.

It is not to be confused with nearby Ramsey County Road 51 (Lexington Avenue / Parkway), which runs parallel to State Highway 51 for much of its route.

Highway 51 begins at its intersection with State Highway 5 (W. 7th Street) in the "West Seventh" neighborhood of Saint Paul. 51 follows Montreal Avenue for 10 blocks. 51 then leaves Montreal Avenue and turns north on Snelling Avenue. Highway 51 remains on Snelling Avenue for the remainder of its route. It continues through the Highland Park / MacGroveland / Merriam Park areas to its junction with Interstate 94/US Highway 52. The route then passes Allianz Field and continues through the Midway neighborhood. After crossing over two railroad lines and entering Falcon Heights, the route passes along the east side of the Minnesota State Fairgrounds, making it a busy thoroughfare during the State Fair season. The route continues north into Roseville and Arden Hills. Most of Highway 51's length through Roseville and Arden Hills is an expressway (divided highway) with signalized intersections. Highway 51 does have an interchange with State Highway 36 in Roseville. The northern terminus of Highway 51 at I-694 in Arden Hills is also shared with the westbound interchange for U.S. Highway 10.

From south to north, the route adjoins the campuses of Macalester College, Hamline University, University of Northwestern - Saint Paul, and Bethel University.

Como Park, Zoo, and Conservatory in Saint Paul is located 7 blocks east of the junction of Highway 51 (Snelling Avenue) and Como Avenue. Eastbound Como Avenue becomes Horton Avenue. The park entrance is on Horton Avenue near Lexington Parkway.

Rosedale Center and Har Mar Mall shopping centers are located on the route in Roseville.

Highway 51 parallels State Highway 280 through Falcon Heights and the Midway neighborhood of Saint Paul.

State Highway 51 was authorized in 1933.

The route was paved by 1935.

The original alignment on the south end of Highway 51 was from West 7th Street up Edgecumbe Road to Snelling Avenue. This section of Highway 51 was later rerouted to Montreal Avenue.

From 1934 to 1935, the north end of Highway 51 previously followed Snelling Avenue northbound to County Road B in Roseville, then 51 turned westbound to Fairview Avenue and ran northbound along Fairview Avenue and New Brighton Road to old U.S. Highway 8 in New Brighton.

By 1935, Snelling Avenue was extended north to the point where Highway 51 now bends to follow Hamline Avenue after crossing the Roseville / Arden Hills boundary line. The northern terminus of the route was at old U.S. Highway 10, which is the present day junction of Snelling Avenue and County Road E in Arden Hills. There was a plan until the 1970s to extend Highway 51 beyond its current northern terminus at I-694 in Arden Hills to proceed north to I-35W near Lexington Avenue in Blaine.

The section of Highway 51 in Roseville and Arden Hills was built as a divided highway by 1970.

The entire route is in Ramsey County.