The first section of the line's western branch dates back to 1963 when an extension, named the University Subway, was built to curve north from Union Station below University Avenue and Queen's Park to near Bloor Street, where it turned west to terminate at St. George station at St. George and Bloor Streets.

[4] In 1978, a further eight-stop extension, dubbed the Spadina Subway, opened, bringing the western leg to Wilson station in North York (then a borough separate from Toronto) primarily along Allen Road.

In 1996, a short 2-kilometre (1.2 mi) one-stop extension to Downsview station (renamed Sheppard West in May 2017) opened.

[9] In 1992, as an interim project, construction began on the aforementioned 1996 extension of the Spadina line to Downsview station.

[5] In 1994, an environmental assessment for the first phase of the loop project was completed, but the extension was not built due to lack of funding.

[9][13] In March 2006, the provincial government announced $670 million in funding for the extension, with the caveat that the line would also be extended to serve the future Vaughan Metropolitan Centre.

[14] At the time, the area around the mostly vacant Vaughan Metropolitan Centre site was occupied by big-box stores and freeways, and lacked the dense development that surrounds most other subway stations.

This included integrated design by architects, public art and stations to meet Toronto Green Standards.

"[20] In 2003, a temporary busway was planned between Downsview (now Sheppard West) station and the campus, but was opposed by the university, which felt it would lessen government willingness to extend the subway.

[32] The TTC later reported that 1,400,000 cubic metres (49,000,000 cu ft) of material had been excavated, with over 54,000 concrete tunnel segments installed.

[33] In November 2014, the TTC indicated that the 2016 opening date would likely be missed, following poor performance by some contractors, death of a worker at the York University station site and harsh winter weather.

Vaughan Metropolitan Centre is also an interchange between the subway and the Highway 7 Rapidway, used by YRT's Viva and Brampton Transit's Züm services.

The bus rapid transit dedicated on-street right-of-way opened in conjunction with the subway, and there is direct escalator and elevator access connecting the station concourse with the covered rapidway stop.

This practice is in contrast to TTC-operated bus routes that cross the Toronto–York boundary at Steeles Avenue, where a second fare is charged.

This is similar to the policy in 1968, when five subway stations opened outside the pre-1998 Toronto city limits in what was the TTC's "Zone 2" area at the time but no extra Zone 2 fare was required to reach those stations, whereas extra fare was charged to connect with suburban TTC bus routes in the boroughs of Metropolitan Toronto.

[66] For a time after the opening of the extension, Downsview Park replaced Bessarion as the least used subway station on the TTC.

[68] By 2019, ridership at Highway 407 had increased dramatically to over 13,000 after GO Transit stopped serving the York University campus and moved their bus connections there.

Finch West and Pioneer Village both saw approximately 17,000 customers and the terminus, Vaughan Metropolitan Centre, had a daily usage of 14,800.

[69] TTC ridership information from 2023 to 2024 showed that the average number of boardings per day was 5,600 at Downsview Park and 7,600 at Highway 407.

[70] However, as a result of the impact of the COVID-19 pandemic and further regional bus route changes,[71] ridership numbers mostly declined or shifted, and the system median itself dropped significantly from 2019.

The busiest stations on the extension in 2024 were York University and Vaughan Metropolitan Centre, with around 20,400 daily riders each.

However, Globe and Mail columnists wrote in the context of the TYSSE: "But that advice will have to compete with the growing power of the suburbs, and the eagerness of Queen's Park to court them."

[15] Vaughan Councillor Alan Shefman stated in 2016 that the new city centre will eventually create the density to justify a subway.

[77] York Region, in their 2022 Transportation Master Plan, proposed further extensions of Line 1 on both the University and Yonge legs.