The ministry replaced the Miller ministry following the 1985 Ontario general election, which resulted in the Progressive Conservative Party of Ontario winning a plurality of seats, but ultimately failing to secure the confidence of the legislature.

The Peterson ministry underwent a significant restructuring following the 1987 Ontario general election, in which the Ontario Liberal Party improved from minority status to a majority, securing 95 out of 130 seats.

None of the incumbent cabinet ministers lost their seats in the election, though three were dismissed by Peterson to the backbenches.

Thus, as the election of 1990 approached, the Peterson ministry contained 25 cabinet members in 29 portfolios.

Alas, the Liberal Party lost 59 seats, falling to 36, surrendering the majority to the NDP, and the Peterson ministry was forced to resign.