He ran for the provincial legislature in a by-election for the riding of Wentworth held on April 5, 1979. He lost to New Democratic Party candidate Colin Isaacs by 543 votes. Two years later, he defeated Isaacs by 260 votes in the 1981 provincial election. Dean was named a minister without portfolio in Davis's government on July 6, 1983. He was appointed Provincial Secretary for Social Development on December 23 of the same year. He supported Dennis Timbrell to succeed Davis in the Progressive Conservative Party's 1985 leadership convention, and was retained in his portfolio when Frank Miller replaced Davis as Premier of Ontario on February 8, 1985.