The M9 and M21 are operated by the New York City Transit Authority, and based out of the Michael J. Quill Depot.

The route runs mostly along Essex Street and Avenue C from Battery Park City to Kips Bay.

Service was suspended, but brought back in March 1929 by the Hamilton Bus Company.

Both the Avenue C and Houston Street segments were served by a single route, the M21, until June 2010.

In 2010, the M9, which ran to 14th Street-Union Square via Avenue B and north of Houston St, was rerouted to Avenue C up to 23rd Street only due to a budget crisis and was rerouted back to Park Row to replace the M15 to City Hall.