Terence O'Neill was born on 10 September 1914 in County Antrim. He was ed the son of Capt. Arthur O'Neill, the first MP to be killed as a result of World War 1. O'Neill was educated at Eton College and then joined the army. During World War 2 he served in the Irish Guards. In a by-election in 1946 he was elected as a Unionist MP for the Bannside constituency in the Stormont parliament.
O'Neill served in a series of junior postions. He was Parliamentary Secretary to the Ministry of Health and Local Government from February 1948 until November 1953, when he was appointed Chairman of Ways and Means and Deputy Speaker of the House of Commons. He was Minister of Home Affairs from April to October 1956 when he was appointed Minister for Finance. In 1963 he succeeded Brookeborough in becoming Prime Minister. He introduced new policies that would have been unheard of with Brookeborough as Prime Minister. He aimed to end sectarianism and to bring Catholics and Protestants into working relationships. He also had great aspirations in the industrial sector. In January 1965 O'Neill invited the Taoiseach of the Republic of Ireland, Sean Lemass, for talks in Belfast. O'Neill met with opposition from his own party mainly because he informed very few of the visit and from Ian Paisley, who rejected any dealings with Dublin. In February O'Neill visited Lemass in Dublin.
In 1968 the civil rights campaign began in Northern Ireland. These peaceful demonstrations were met with violence from the police force and the IRA quickly began recruiting again. In February 1969 O'Neill called a surprise general election. The electorate were faced with a choice, however the election results were inconclusive. O'Neill resigned as leader and retired from Stormont politics in January 1970 when he resigned his seat. In that year he was made a life peer (Lord O'Neill of the Maine) in the House of Lords.