The L Train route was mostly built by the Brooklyn Rapid Transit (BRT) corporation and its successor, the Brooklyn Manhattan Transit (BMT) corporation in the early 1900s through the early 20s. The earliest part of the current L Train line to be constructed was a portion at the Canarsie end, which was part of a late 19th century Long Island Railroad steam line. This was later ceded to the BRT and incorporated into a north Brooklyn transit line.

This line ran at grade for the portion of the old steam line from Canarsie, then connected into the Broadway elevated (today's J, M, and Z Trains) at the Atlantic Avenue/Eastern Parkway complex for service to Williamsburg and Manhattan over the Williamsburg Bridge. The track connections are still there, although regular no longer operates on them. In 1924, as part of the dual contracts era of subway construction, the BRT completed its second subway tunnel under the East River, connecting 14th street in Manhattan with Williamsburg. This subway ran from Union Square and extended several stops into Williamsburg and Bushwick and across North Brooklyn, connecting into the Canarsie line and providing an additional route to Manhattan.

In the Early 30s, in some of the last BMT construction before consolidation by the city, the Manhattan portion of the line under 14th street was extended from Union Square to Eighth Avenue, providing transfer connections to the new city built Independent (IND) Sixth Avenue and Eighth Avenue lines (today's A, C, E, and F Trains). This portion was rather unique for BMT trackage in that the stations were built in the no frills IND style. The line that is today's L thus assumed its present form at this early date, running from Canarsie through the North Brooklyn subway to Williamsburg, under the East River and under 14th street in Manhattan to Eighth Avenue.

In 1940, the BMT was taken over and the city assumed operation of all transit lines in New York City. Regular service from Canarsie was discontinued over the afore mentioned Broadway elevated, and all service proceeded through the subway. This line was assigned the letter designation L, and is one of the only former BMT routes not to be effected by consolidation.

Until 1984, the L line had one grade crossing of a street, the only such place on the New York City subway. In that year, the grade crossing was replaced with a bridge.

The L line currently runs a mix of R40 and R42 rolling stock, cars from the early 60s that are nearing the end of their useful lives. Supposedly the L will receive some of the first R143 cars in the next year or so, the next generation of high tech BMT/IND subway cars.

Supposedly the L is the most crowded rush hour route. This is probably caused by service patterns not keeping up with the growth of Williamsburg, although it is a feat considering the epic crowding on all the other routes.