As most of the readily developed hydroelectric sites became exploited, the corporation expanded into building coal-fired generation and then nuclear-powered facilities.

[1] Starting in 1899, several private syndicates sought privileges from the commission for generating power for sale, including:[2] By 1900, a total capacity of 400,000 horsepower (300,000 kW) was in development at Niagara, and concern was expressed as to whether such natural resources were being best exploited for the public welfare.

Spence, who recommended in February 1903 that authority be sought from the Ontario Legislature to allow municipal councils to organize a cooperative to develop, transmit, buy and sell electrical energy.

[4] The provincial government of George William Ross refused to allow this, and it was only after its loss in the 1905 election that work began on creating a public utility.

During that election campaign, James Pliny Whitney (who would become Premier) declared: The water power of Niagara should be as free as the air.

[10] The victories in Toronto were in large part due to the leadership and commitment of Adam Beck's ally, William Peyton Hubbard.

Berlin (Kitchener) would be the first city in Ontario to get hydroelectric power in long-distance transmission lines from Niagara Falls, on October 11, 1910.

[12] The commission's process of expansion was from municipality to municipality, generally in the following manner:[13] During the 1920s, Hydro's network expanded significantly: In 1926, the Ferguson government gave its approval for Abitibi Power and Paper Company to develop the Abitibi Canyon, the largest such development since the Niagara River, in preference to incurring more debt for Ontario Hydro.

[a] Certain dealings relating to the 1933 acquisition came to be known as the "great Abitibi swindle,"[21] which resulted in the fall of the Henry government in the 1934 Ontario election, to be succeeded by that of Mitchell Hepburn.

As demand rose in the post-war period, Ontario Hydro started expanding its generation system bringing on line many new hydroelectric stations.

[24] Changes in government policy and public sentiment in the 1920s restricted their development, and all such operations ceased in the 1930s (with the exception of the Hamilton Street Railway streetcar system, which continued until 1946).

In 1922, Dougall Carmichael, then Minister without Portfolio, announced to the Legislature that he was quitting his position as Commissioner because Hydro "was either inefficient or dishonest."

[29] In the 1970s, controversy arose relating to Hydro's expansion strategy, and several inquiries were held: In the 1980s there were large increases in the rates charged, arising from: In 1989, Ontario Hydro published a four-volume study, forecasting up to the year 2014, entitled Providing the Balance of Power, with different scenarios attempting to address the need for additional facilities to replace aging electricity generation stations.