He was a member of Sligo Borough Council (formerly Sligo Corporation) from 1999 to 2014, having been elected on three occasions, and was a member of Sligo County Council from 2017 to 2020. He was co-opted in March 2017 to replace his father Seán MacManus, and retained the seat at the 2019 Sligo County Council election.
MacManus sits on the Agriculture and Rural Development Committee as well as the Committee on Economic and Monetary Affairs. In the former capacity, he is a shadow rapporteur for The Left on the 2020/21 reform of the Common Agricultural Policy.
He is also a member of the EU Delegations for relations with South Africa and with Palestine.