Construction of an extension north to York Regional Road 25 (Major Mackenzie Drive) began in 2017 and was opened on September 18, 2021.

[2] At its southern terminus, the route begins at Coules Court, where it continues as Brown's Line, once the southernmost stretch of Highway 27.

[6] After crossing Canadian Pacific Railway (CPR) tracks, the freeway interchanges with Dundas Street (former Highway 5).

A set of criss-crossing ramps provide access between the collector and express lanes north of this point,[6] referred to as "The Basketweave", with the northbound express-to-collector transfer also having an offramp to The East Mall and Dundas Street.

[6] Transfers provide a second and final opportunity to cross between the express and collector lanes, or vice versa, south of the complicated 1.56-square-kilometre (0.60 sq mi) Highway 401 interchange.

[5] Highway 427 crosses Renforth Drive and then curves to the east of Runway 24R and 24L of Toronto Pearson International Airport.

[5] After crossing the Kitchener GO line, it passes west of Woodbine Racetrack and interchanges with Rexdale Boulevard/Derry Road and Finch Avenue.

The designation was applied following the completion of the interchanges at the QEW and Highway 401 as well as the expansion of the section between them into a collector-express system.

The remainder was designed to follow the existing right-of-way of Highway 27 between the QEW and Richview Sideroad (now Eglinton Avenue).

[24][1][25] Replacing a cloverleaf, the new interchange with the QEW was built over 48.5 ha (120 acres) and required the construction of 19 bridges and the equivalent of 42 km (26 mi) of two-lane roadway.

An extension north of Dixon/Airport Roads began in 1976 as part of the work to build Highway 409,[32] and it included the construction of the interchange between the two freeways.

By the beginning of 1980, this work was completed, and construction was progressing on the section north to Rexdale Boulevard, which opened by the end of the year.

The extension followed a new alignment since this has sufficient right-of-way for future expansion and a junction with the initial phase of Highway 407, as opposed to upgrading Indian Line and incorporating it into the freeway.

The bypassed remanent of Indian Line ending at Albion Road was then closed off to regular vehicular traffic,[38] though it is still used for utility vehicle access to a natural gas facility.

[39] The final at-grade intersections were removed in the early 1990s, the first being the signalized left turn from the southbound lanes with eastbound Highway 409 which was replaced by a flyover ramp in 1992, and the second being at Morning Star Drive where an overpass was constructed in 1994 to extend the street across the freeway to Humberwood Boulevard, making Highway 427 a fully controlled-access freeway for its entire length.

This included a new loop ramp from the Highway 427 southbound collectors to the Gardiner, aimed at relieving the congestion in the express lanes created by the southbound collector-to-express transfer near Bloor Street, as the collector lanes originally lacked direct access to the Toronto-bound QEW (downloaded from the province in 1998 to become the part of the Gardiner).

The Gardiner Expressway also received an off-ramp to Sherway Gardens, which necessitated an underpass to be implemented in the directional ramp from the Highway 427 southbound express to the Hamilton-bound QEW.

[48] This project included the installation of high-mast lighting, median barriers, and the addition of high-occupancy toll lanes (HOT) in both directions, and was completed in 2021 in conjunction with the Vaughan Extension (see below).

One of the challenges during this project was widening the Highway 427 bridges crossing Highway 407 ETR, with the solution being steel box girders added on either side of the existing post-tensioned concrete structures, as opposed to the conventional bridge widening practice of the expansion using a similar construction to the original bridge since post-tensioned concrete additions require falsework which in turn would close down Highway 407 ETR lanes for extended periods.

An environmental assessment was completed on a northward extension of Highway 427 to Major Mackenzie Drive to relieve traffic issues on Regional Road 27 and 50, as well as provide improved access to the Canadian Pacific Intermodal Terminal,[46] with construction beginning in May 2017.

[52] However, a pending legal dispute between the provincial government and the constructor delayed the opening of the extension in late April 2021.

In the past decade, there has been little discussion of this highway extension due to concerns with traversing the Oak Ridges Moraine and Minesing Wetlands.

[62][63] The following table lists the major junctions along Highway 427, as noted by the Ministry of Transportation of Ontario.