The facility was constructed in stages between 1965 and 1986[5] by the provincial Crown corporation, Ontario Hydro, with significant completion of Station A scheduled for 1971.

On December 31, 1997 the four Pickering A reactors, along with the remaining three units at Bruce A, were shut down by Ontario Hydro for safety reasons[15] and placed in lay-up.

Specific to Pickering A, four years earlier the AECB had required mandatory upgrades to the safe shutdown system be completed by the end of 1997,[16] which differed from that at the other three plants.

Pickering A featured a moderator dump as its 2nd shutdown system,[17] and this was deemed too slow compared to the poison injection system that later plants used, including Pickering B. Ontario Hydro committed to the refit and restart project, but it underwent long delays and large cost over-runs.

Instead of retrofitting the poison injection found at the other plants, the least cost option was to add more shutdown rods and then split them into separate, independent groups.

This was deemed sufficient by the AECB, despite acknowledging that this does not in fact constitute a fully independent fast acting secondary safe shutdown system.

[18] Premier Mike Harris asked former federal energy Minister Jake Epp to study and make recommendations on the problems with the Pickering restart.

[19] Mr. Epp and the Pickering A Review Panel released their report in December 2003,[20] which acknowledged the large cost over-runs and delays, attributing blame to bad management.

The anti-nuclear group Sierra Club of Canada criticized the 2004 OPG Review Committee report for not attributing any blame to the problems of nuclear technology, noting that there were no energy or environmental experts appointed to the panel.

[31] - In 2019, Pickering set a site capacity factor record of 87.07%, producing 23.6TWh and putting it roughly on-par with the much newer Darlington and Bruce facilities.

[2] In January 2016, the Province of Ontario approved plans to pursue continued operation of the Pickering Nuclear Generating Station to 2024.

[33] By 2016, OPG had begun planning for the end of commercial operations at the generating station, including the potential repurposing of the Pickering site location.

Simultaneously, it announced that it had requested OPG to update feasibility studies on the potential refurbishment of the four units of Pickering B.

In its announcement, the Province stated that continued operation of the station would reduce carbon dioxide emissions by 2.1 megatonnes in 2026, as well as increasing the North American supply of cobalt-60, a medical isotope.

[35][36] In August 2023, the OPG Board of Directors agreed with and authorized the submission of the feasibility assessment for the refurbishment of the Pickering B plant to the province as well as to proceed with preliminary planning and preparation activities for the project.

Since then, careful monitoring of the location of the annulus gas spacer rings has been a significant part of routine reactor inspections.

An Independent, Integrated Performance Assessment report noted that Pickering stations A and B were cited for breaking regulation 15 times and having 13 fires for the year.

"[46] On March 14, 2011, there was a leak of 73 cubic metres of demineralized water into Lake Ontario from a failed pump seal.

[48][49] Solicitor General Sylvia Jones stated that the alert was accidentally issued during a "routine training exercise" by Ontario's emergency operations centre.

[50] The false alarm also prompted renewed interest in preparedness for actual nuclear accidents: OPG reported a surge in the sales of potassium iodide kits via its "Prepare to Be Safe" website between January 12 and 13, increasing from its monthly average of 100 to 200, to over 32,000.