It is a Crown agency that manages and integrates road and public transportation in the Greater Toronto and Hamilton Area (GTHA).

Metrolinx serves as the central procurement agency on behalf of Ontario municipalities for local transit vehicles, equipment and services.

In April 2007, a transition team seconded from the Ontario Public Service began work at the GTTA's headquarters at 20 Bay Street in Toronto.

The Metrolinx Investment Strategy, released in May 2013, proposes a series of 24 recommendations as part of a four-part plan to integrate transportation, growth and land use planning in the GTHA, maximize the value of public infrastructure investment, optimize system and network efficiencies, and dedicate new revenue sources for transit and transportation.

Canada's first such public system, GO Transit began regular passenger service on May 23, 1967, under the auspices of the Ontario Ministry of Transportation.

[134][133] Since June 2013, GO Trains along the Lakeshore rail lines run every 30 minutes, making the biggest expansion in GO Transit history.

The line uses a Metrolinx-owned railway rail corridor now used by GO Transit, as part of the Georgetown South Project to allow for additional train traffic.

The UP Express shares the same path as trains on the Kitchener line, before splitting off onto a separate subdivision just west of the Etobicoke North Station.

Employers and employees in the GTHA can explore and have assistance with different commuting options, such as carpooling, transit, cycling, walking, remote work, and flextime.

[142] Metrolinx was again criticized when, in January 2012, its CEO declared that it would bend to what Toronto City Council wanted regarding how the Eglinton-Scarborough Crosstown LRT line should be built.

The issue centred on whether the more suburban stretches of the line, from Laird Drive to Kennedy Station, should be built at street level instead of a more costly underground alignment.

[143] Metrolinx was criticized after a Toronto Star investigation found that the agency has approved two transit stations, Kirby and Lawrence East, for the GO Regional Express Rail expansion due to political pressure from the Ministry of Transportation.

Kirby is in the Vaughan riding of the then-transportation minister, Steven Del Duca, and Lawrence East in Scarborough is part of Toronto mayor John Tory's "SmartTrack" plan, his signature campaign promise.

[144][145] Ontario's auditor general found that Metrolinx incurred about $436 million "in sunk and additional" – unrecoverable – costs between 2009 and 2018 due to numerous changes in transit plans.