Beattie grew up in Ballymena. In 1965, he became a student minister at the Dunmurry Free Presbyterian Church, and in 1967 he became a full minister in the Church, led by Ian Paisley. He also joined Paisley's Protestant Unionist Party (PUP), and became the deputy leader.
In 1970, Beattie was elected to the Northern Ireland House of Commons in a by-election in South Antrim. On the same day, Paisley was elected for Bannside, and the two became the PUP's first Members of Parliament.
In 1971, the PUP formed the Democratic Unionist Party (DUP), and Beattie retained his post as deputy leader. The Parliament was prorogued in 1972, but Beattie was elected to the Northern Ireland Assembly of 1973, and became deputy chief whip of the United Unionist Assembly Party. He was again elected, to the Northern Ireland Constitutional Convention in 1975, to Lisburn District Council in 1977, and to the Northern Ireland Assembly of 1982.
Beattie stood in Lagan Valley in the 1983 general election, but again failed to win a seat in the British House of Commons. He resigned from the DUP in the mid-1990s, and retired as a minister on 31 December 2005.