In May 1940 he received a commission at the Royal Military College, Sandhurst,[1] and went on to serve in the 6th Guards Tank Brigade during the Second World War, in which both of his brothers died.

[2] On 4 February 1944 he married Katharine Jean,[3] the daughter of William Ingham Whitaker, of Pylewell Park, Lymington, Hampshire.

[4] At the end of 1945, O'Neill and his family went to live in Northern Ireland in a converted Regency rectory near Ahoghill, County Antrim.

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 of Northern Ireland.

[6] In 1963, O'Neill succeeded Basil Brooke, 1st Viscount Brookeborough as Prime Minister of Northern Ireland and Leader of the Ulster Unionist Party.

[7] As O'Neill promoted industrialisation and modernisation, Taoiseach Seán Lemass was doing similar things in the Republic of Ireland, thus leading to the first real rapprochement between the two jurisdictions since partition.

O'Neill met with strong opposition from his own party, having informed very few of the visit, and from Ian Paisley, who rejected any dealings with the Republic.

Opposition to O'Neill's reforms was so strong that in 1967 George Forrest – the MP for Mid Ulster, who supported the Prime Minister – was pulled off the platform at the Twelfth of July celebrations in Coagh, County Tyrone, and kicked unconscious by fellow members of the Orange Order.

On 19 January 1968, O'Neill made a speech marking five years in office to members of the Irish Association, calling for "a new endeavour by organisations in Northern Ireland to cross denominational barriers and advance the cause of better community relations".

The march across Derry on 5 October 1968, banned by William Craig the Minister of Home Affairs, was met with violence from the Royal Ulster Constabulary (RUC) who used batons on protesters, among whom were prominent politicians.

This granted a number of the concessions that NICRA had demanded but importantly it did not include one man one vote in local government council elections.

A group was formed by university-based activists including Bernadette Devlin and Michael Farrell, named People's Democracy, which began a four-day march from Belfast to Derry on 1 January 1969.

Although pro-O'Neill candidates won a plurality of seats in the general election, O'Neill lost an overall majority among UUP MPs in order to pass his reforms through Parliament.

He was humiliated by his near-defeat in his own constituency of Bannside by Ian Paisley and resigned as leader of the UUP and as Prime Minister on 28 April 1969 after a series of bomb explosions on Belfast's water supply by the Ulster Volunteer Force (UVF) brought his personal political crisis to a head.

O'Neill spent his last years at Lisle Court, Lymington, Hampshire, although he continued to speak on the problems of Northern Ireland in the House of Lords where he sat as a cross-bencher.