He was born Aughnasheelan, Ballinamore, County Leitrim, the son of small farmer Francis McCartin and his wife Annie Kate Lohan. Educated at Drumbibe National School, Aughnasheelan and St. Patrick's College, Cavan. In 1972 he married Ann Clarke. They have two sons. As a young farmer, McCartin was awarded many national prizes for progressive and efficient farming. He founded the McCartin Bros group of companies with his brother Tommy involving Structural Steel, Clothing Manufacture, Milling, Forestry, Pig Production, and Dairy/Beef Farming. They quickly became the largest employers in Leitrim.
McCartin joined Fine Gael as a teenager and became secretary of the Ballinamore District Executive at the age of seventeen. He served as county secretary of Macra na Feirme (Young Farmers Organisation) and was a member of its national executive. 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.
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. He retired form the European Parliament in 2004 having been Ireland's longest-ever serving Member of the European Parliament (MEP).
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".