MacDonell first attempted to enter politics at the 1959 local-body elections when he stood unsuccessfully for the
Dunedin City Council on the Labour Party ticket. He stood for the council again in 1962 and was likewise unsuccessful. He represented the
Dunedin Central electorate in Parliament for 21 years from to 1984. He was Parliamentary Under-Secretary to the Minister of Trade and Industry (1973–1975) and to the Minister of Energy Resources (1974–1975). Following the
Royal Commission on Contraception, Sterilisation and Abortion, parliament discussed legislation to legalise abortion, and MacDonell supported his pro-life stance by holding a jar with a pickled 12-week-old foetus during the debate. Parliament passed the
Contraception, Sterilisation, and Abortion Act 1977. MacDonell was Shadow Minister of Customs and Shadow Postmaster-General from 1975 to 1983 under
Bill Rowling. The Dunedin Central electorate was abolished in the 1983 electoral redistribution, and the electorate of
Dunedin West was established in its place. MacDonell was not selected as the Labour candidate for the new electorate of Dunedin West; instead, the party's president,
Jim Anderton, installed his personal friend
Clive Matthewson. MacDonell left the party and became an
Independent. He worked closely in the house with another Labour MP turned independent,
John Kirk, as well as the two
Social Credit Party's MPs
Bruce Beetham and
Gary Knapp. The quartet appointed MacDonell as the groups
whip to boost their recognition in parliament. At the
1984 general election he was not successful standing for re-election. MacDonell was later elected a member of the
Dunedin City Council at the 1995 local-body elections representing the South Dunedin ward for three years. ==Later activities==