At the 1977 general election he was elected to Dáil Éireann as a Fine Gael TD for the Dublin Ballyfermot constituency. With the party's loss of power in 1977, the new leader, Garret FitzGerald appointed Mitchell to the Party's Front Bench as Spokesperson for Labour. At the 1981 general election, Mitchell was elected for the Dublin West constituency and Fine Gael dramatically increased its number of seats and formed a coalition government with the Labour Party. On his appointment as Taoiseach, Garret FitzGerald caused some surprise by excluding some of the older conservative former ministers from his cabinet. Instead young liberals like Mitchell were appointed, with Mitchell receiving the high-profile post of Minister for Justice. The Fine Gael–Labour government collapsed in January 1982, but regained power in December of that year. Mitchell again was included in a FitzGerald cabinet, as Minister for Transport. He also served as Minister for Posts and Telegraphs (later titled Minister for Communications) from 1982–87.