Howlin has been a TD since first elected to the Dáil in 1987. He has retained his seat at every election since. He previously served in the Seanad from 1983 to 1987. He has been the leader of the Labour Party since 2016. Previously he was the deputy leader of the party. He has served as Minister for the Environment and Health, respectively. He is a former councillor on Wexford County Council, holding his seat from 1985 to 1993.