Wellesley station opened in 1954 as part of the original stretch of the Yonge line from to stations, and was listed as a heritage property by the City of Toronto in 1984. The address given for this listing of 16 Wellesley Street East/15 Dundonald Street, which differs from the
Toronto Transit Commission (TTC) address, more accurately describes the location of the station structure which runs between those streets, to the rear of the properties fronting on the east side of Yonge Street. On February 6, 2006, the only baby to be born on the subway, Mary Kim of
Scarborough, was delivered on the Wellesley platform. Her mother Sun Hee Paik took the subway with her family to
St. Michael's Hospital from their Scarborough home. She did not make it to the downtown hospital, going into labour on the train. Her husband delivered the baby after they disembarked at Wellesley and
Toronto EMS arrived later to help finish the birth and send the mother and child to St. Michael's. TTC officials later promised to provide Mary with lifetime transit access. Construction began in early 2018 to make the station fully accessible, including the addition of two elevators and accessible fare gates. On July 22, 2020, the station became accessible with the completion of the elevator construction. As part of the enhancements for accessibility, provision was made for the future installation of an artwork on the curved wall opposite the two new elevators.
Second exit Wellesley was the last downtown TTC subway station with only one street entrance. Plans for a second exit to be located on Wellesley street were tendered in 2011. Construction work for the portion internal to the station began in the fourth quarter of 2014. The projected completion date was initially the second quarter of 2017 but the work was not completed for years after. The mezzanine level leads to two separate exits on Dundonald Street, at 17 Dundonald and at the residential entrance of 597-599 Yonge Street. The TTC claimed that it has completed all work within its control by mid-2017, and that the opening had been delayed by the construction of two condominium buildings. However, despite residents having moved into 17 Dundonald in the second quarter of 2018, the station entrance in the building was not opened until July 27, 2020. Residents of 599 Yonge Street began to take residency in late 2022. As of January 2025, the entrance in that building remains closed. ==Station description==