The cities of Mississauga and Brampton have determined that rapid transit along Hurontario is required due to the chronic overcrowding of Mississauga's (and the suburban Greater Toronto Area's) busiest bus routes, 2/17 Hurontario, which carry more than 25,000 passengers a day, combined with the numerous high-density development proposals along the corridor and the high growth in both cities.

[15]) On April 21, 2015, the Government of Ontario announced that it would completely fund the line, not including local capital costs such as utility relocations, surface upgrades, and landscaping.

According to then-Mayor Bonnie Crombie, Mississauga is planning for mixed-use zoning along Hurontario including accommodation, businesses, commercial, retail and arts-cultural development.

[27] On October 28, 2015, Brampton City Council voted 7–4 against allowing the LRT to run along Main Street through its heritage downtown area, as originally planned by the province.

[8] Opposed council members had also previously cited a lack of projected growth along the northern half of the proposed Brampton route to support an LRT.

Councillors opposing the Main Street route have proposed running the LRT east or west along Steeles Avenue and then north to Queen Street where it would then possibly continue east from Brampton's downtown area to the Bramalea GO Station or possibly all the way to the terminus of the western branch of the Toronto subway's Line 1 Yonge–University at Vaughan Metropolitan Centre.

[32] Metrolinx CEO Bruce McCuaig said the provincial money allocated to the Main Street route in Brampton would now be available for other transit projects across the province.

[8] On November 3, 2015, Transportation Minister Steven Del Duca announced that the funding for the cancelled Main Street route will be invested in priority transit projects in the Greater Toronto Area which might or might not include Brampton.

[33] At its February 20, 2020, meeting, the Metrolinx Board of Directors endorsed a prioritization framework for a proposed Frequent Rapid Transit Network that was inclusive of a reinstated LRT extension from Brampton Gateway Terminal to Brampton GO Station; with a forecasted ridership of 5,500 in 2031 and a proposed line length of 3.3 km (2.1 mi) along Main Street, the project scored 'medium' with a preliminary benefit-cost ratio of 0.66–0.90.

City Council approved the recommendation to progress the LRT extension study to 30% Preliminary Design and to prepare a Draft Environmental Project Report for both alternatives.

City staff still recommend the underground alignment due to perceived benefits in travel time savings, infrastructure modifications, downtown revitalization, operations and maintenance, and protection for a future extension.

The loop would serve most of the City Centre at a walking distance of 500 metres (1,600 ft), and include stops on Burnhamthorpe and Rathburn Roads, and a still-undecided north–south street.

It was initially cancelled on March 21, 2019, when Metrolinx announced that the downtown loop would be dropped due to financial restrictions beyond the spur to the stop on Rathburn serving the City Centre Transit Terminal.

[39] On February 14, 2022, at the aforementioned press conference to rename the line after former Mayor Hazel McCallion, Premier Doug Ford offered to reinstate the loop but did not specify a time frame.

[23][40] By January 18, 2024, the Ontario Minister of Transportation had requested that Metrolinx provide an initial business case by February 5 for constructing both the Main Street extension and the Mississauga City Centre loop.

[41] Three weeks later, on February 8, the Ontario government approved both projects, but did not specify stop locations for either or if the Main Street extension would be tunnelled or at-grade (either as LRT or a mixed-traffic streetcar) through Brampton's downtown.

[7] At a 2013 open house, local residents objected to this extension as an invasion of a residential area and an inconvenience for pedestrians and motorists using the street.

However, by January 2022, Brampton reversed its decision and requested that the stop be moved back to the north side so that riders would not have to cross Steeles Avenue to transfer between the LRT and the bus terminal.

[47] In 2010, Metrolinx placed an order for 182 Flexity Freedom vehicles manufactured by Bombardier for use on the light rail lines it was building in Greater Toronto.

[49] On October 18, 2016, IO and Metrolinx started the procurement process by issuing a request for qualifications to design, build, operate and maintain the Hurontario LRT.

[54] On May 23, 2019, IO and Metrolinx announced that proposal have been submitted by only two of the three shortlisted teams of private companies, namely Mobilinx and Trillium Transit Partners.

[55][56] On October 21, 2019, IO and Metrolinx announced that Mobilinx had been awarded the contract to design, build, finance, operate, and maintain the Hurontario LRT for a period of 30 years.

[59] This work was completed by June 2023, with the push box ready to be converted into a 46-metre (151 ft) tunnel under the GO railway tracks for the LRT line.

[65] Track construction started in September 2022 at various intersections along Hurontario Street between Sandstone Drive (south of Britannia Road) and Matheson Boulevard.

[61][67] On March 8, 2023, the first of 13 traction substations to convert from AC to DC and to provide electricity for the trains, each housed within a steel structure about the size of a shipping container, was installed at Skyway Drive and Hurontario Street.

Further north, between the Robert Speck stop and Highway 403, the line will switch to the west side of Hurontario Street and run onto an elevated guideway.

[73] The main building will have an area of 10,634 square metres (114,460 sq ft) and contain five through tracks, a vehicle repair shop, a cleaning facility and material storage.

Its local counterpart, 19 Hurontario, was cut to GO Transit's Highway 407 Park and Ride to fortify the overlapping express services, however its frequency was further increased to address ongoing overcrowding issues between Britannia and Lakeshore Roads, the busiest section of the corridor.

In 2013, Brampton mayor Susan Fennell had proposed to run the 502 Züm Main along the entire LRT route to Port Credit GO as a temporary measure.

Then-Mississauga Mayor Hazel McCallion rejected the alternative proposal, citing gridlock south of Mississauga City Centre as a reason.