[2] A 2018–19 MBTA review of its bus system found that route 86 had infrequent and unreliable service, including irregular scheduled headways, despite its high ridership and significance as a crosstown connecting route.

Transit service on portions of the modern route date back to horsecar lines opened in the 19th century, several of which were converted to electric streetcars in the 1890s.

Three overlapping bus routes (later designated 63, 86, and 91) were established between 1925 and 1931, replacing circumferential streetcar services and creating new connections.

[2] Several portions of the modern iterations of the route originated as horsecar and streetcar lines: Horsecar and streetcar operations in Boston and surrounding municipalities were consolidated under the West End Street Railway in the 1880s; it was leased by the Boston Elevated Railway (BERy) in 1897.

[7]: 46  A Harvard Square–Union Square, Somerville bus route began operation on August 22, 1925.

[15] On November 15, that shuttle was replaced with a new bus route at the city's request, with the intention of reducing congestion in Union Square.

[19]: 143 On June 8, 1931, the Brighton Center–Central Square streetcar line was replaced with buses and extended south to Commonwealth Avenue.

[21] (The Western Avenue tracks would remain in use until 1950 for Watertown service, which is now bus route 70.

The Western Avenue segment was resumed in 1946, forming a Cleveland Circle–Central Square route.

[24] On July 4, 1959, the southern terminal was extended slightly from Cleveland Circle to a new busway at Reservoir station as the Riverside Line opened, though the terminus was still listed as "Cleveland Circle" in public schedules.

[25] In September 1962, the MTA through-routed several routes as part of experimental attempts to increase ridership.

[24] The MTA was in turn replaced by the Massachusetts Bay Transportation Authority (MBTA) in 1964.

Elderly patrons who lost their one-seat ride from Brighton to Central Square petitioned for route 63 to return.