The Toronto Civic Railways opened the St. Clair streetcar route in 1913 along St. Clair Avenue West between Yonge Street and the Grand Trunk Railway crossing (near today's Caledonia Road) to serve small developed areas in a newly annexed section of the city.

Furthermore, St. Clair Avenue is one of the few streets in Toronto wide enough to accommodate a dedicated right-of-way without significantly reducing the width of traffic lanes.

Construction started on September 25, 2005 and was completed in three stages: From St. Clair station to Vaughan Road on February 18, 2007,[17] to Lansdowne Avenue on December 20, 2009,[18] and finally with full service to Gunns Loop on June 30, 2010.

[19][8] On December 19, 2009, the day prior to the opening of the section from Vaughan Road to Lansdowne Avenue, there was a pre-opening event utilizing the TTC's two remaining PCC streetcars.

From July 31, 2005, until September 2, 2017, the TTC ran a pilot project of providing 2-hour time-based transfers[20] on this route as a temporary measure to support business along St. Clair during the construction projects of the new dedicated streetcar right-of-way scheme, under which passengers who took a paper transfer after paying their fares by cash or tokens were allowed to disembark and re-board another 512 streetcar, even one going in the opposite direction, as long as they did so within 2 hours of their original boarding.

This meant that one could stop part-way through a journey and then continue, or even make a round trip, without paying multiple fares.

At the time, the Presto card readers on the 512 cars were not configured to handle this special route-specific time-based transfer pilot scheme.

Presto users who wished to take advantage of the project were required to board at the front door of the vehicle in order to obtain the special paper time-based transfer after tapping their cards on the fare reader there, or else they would have been charged another fare every time they re-tapped on the same route.

[22] About 2007, two plans had been proposed to extend the St. Clair line west of Gunns Loop, but neither is active today.

At the time, transit advocate Steve Munro thought that extensions west of Gunns Loop might lack the ridership to be justified.

[24] According to a 2011 Torontoist article, it took 29 minutes for the author to travel from St. Clair station to Gunns Loop shortly after the rush hour.