He became Prime Minister of Northern Ireland in 1963. He initiated a policy of modernising the economy and administering devolution. He met Seán Lemass at a summit in Stormont in 1965, and Jack Lynch in 1967. Little of substance was agreed, though cross-border cooperation increased, particularly in tourism promotion and electricity supply.
In 1968 he unveiled a programme of reforms, notably the closing down of the gerrymandered Londonderry Corporation. He warned in a television broadcast that “Northern Ireland stood at the crossroads”. In the face of fierce unionist opposition and escalating violence, he resigned on 28 April 1969.