Educated at Drumbibe National School, Aughnasheelan and Saint Patrick"s College, Cavan. They have two sons. They quickly became the largest employers in Leitrim. He was Chairman of the fund raising committee which established the first rehabilitation centre in the North-West of Ireland.
He was elected to Leitrim County Council in 1967.
In 1973 he was elected to Seanad Éireann by the Agricultural Panel, and after his re-election to the Seanad in 1977 he was elected Leas-Chathaoirleach of the 14th Seanad. He was elected to the European Parliament in 1979 for the Connacht–Ulster constituency and to the Dáil at 1981 general election representing the Sligo–Leitrim constituency.
He lost his seat at the February 1982 general election but regained it at the November 1982 general election. He lost it at the subsequent 1987 general election and did not stand in any further Dáil elections.
In the European Parliament, McCartin was elected Vice Chairman of the Parliament"s Budgetary Control Committee.
He was the only Irish person ever to hold the position of Vice President of the European People"s Party. He also served on the parliament"s Agriculture and Fisheries committees. He was Joint Auditor of the European Peoples Party along with John Bruton, former Taoiseach.
In 1974 he was elected Leitrim Person of the Year.
In 1977 he was named as one of Ireland"s "People of the Year" for "bringing hope to a depressed area."
In 2004 he was presented with the Schuman Medal at a meeting of the European Christian Democrats in Budapest, Hungary for "his outstanding contribution to peace and unity in Europe and for the promotion of the human values of mankind".
McCartin joined Fine Gael as a teenager and became secretary of the Ballinamore District Executive at the age of seventeen. He served as secretary of the Fine Gael Parliamentary Party, Secretary of the Opposition Front bench and party spokesperson Minister for Agriculture.
He served as county secretary of Macra na Feirme (Young Farmers Organisation) and was a member of its national executive. He retired form the European Parliament in 2004 having been Ireland"s longest-ever serving Member of the European Parliament (Member of the European Parliament).