Deasy was elected as a Fine Gael member of both Dungarvan Urban District Council and Waterford County Council in 1967, positions he held until 1983. He was chairman of the latter body in 1980-81, before reclaiming his seat on the Urban District Council from 1994 to 1999. Deasy was also a member of the South East Health Board, Waterford Harbour Commissioners and Waterford Vocational Education Committee. He contested the 1969 general election, as a Fine Gael candidate for Waterford. However, he finished fifth in the three-seat constituency. Deasy finished in fifth position again when he stood as a candidate in the same constituency at the 1973 general election. Deasy was selected as a candidate for the Cultural and Educational Panel for the subsequent election to Seanad Éireann in 1973. However, he was once again unsuccessful. In spite of this, he was appointed to the Seanad as one of Taoiseach Liam Cosgrave's nominees.
TD
Deasy contested his third general election in 1977. He was successful in being elected that time, taking the third seat in the newly expanded constituency and finishing ahead of his running mateEdward Collins. Fine Gael returned to opposition following that election, however, Deasy was immediately appointed to the Front Bench as Spokesperson for Fisheries. In a 1979 reshuffle, he became Spokesperson for Transport, Communications and Technology, a position he held until early 1981, when he ceded the Communications and Technology elements of his brief. Following the formation of a Fine Gael-Labour coalition government in 1981, Deasy was one of several Front Bench spokespersons who were unlucky not to be appointed to cabinet or as a Minister of State. The collapse of the coalition government in 1982, saw Deasy return to Garret FitzGerald's Front Bench as Spokesperson on Foreign Affairs. The formation of a new Fine Gael-Labour coalition in 1982, saw Deasy being appointed as Minister for Agriculture. During his four-year tenure in the position he was seen as an effective Minister. In 1988, Deasy resigned from the Fine Gael Party, in protest against Alan Dukes' Tallaght Strategy, in which Fine Gael would support the minority Fianna Fáil government on budgetary related issues. The following year Deasy tried unsuccessfully to remove Dukes as leader. Deasy called the agreement treacherous and said he could support any party who expressed confidence in Fianna Fáil or Charles Haughey. This was widely seen as the actual opinion of a majority of Fine Gael TD's and Senators. When the agreement had broken between Fine Gael and Fianna Fáil, Deasy agreed to return to Fine Gael. Upon his return in 1991, he was appointed to the front bench, after John Bruton became leader. Deasy later resigned again due to the financial difficulties the party was facing. In 2000, Deasy introduced an unsuccessful motion of no confidence in Bruton. Deasy was succeeded as TD by his son, John Deasy.
Deasy was married to Kathleen Keating until his death, and together they had four children: John, Jamie, Sally and Jane. After a short illness, Deasy died at University Hospital Waterford on 10 June 2017, at the age of 80.