McCullough was based on the Shankill Road. He was elected to Belfast City Council for the group in 1958, topping the poll. He left the group before the next elections, in 1961, joining the Ulster Unionist Party (UUP).
Cullough secured re-election under his new party colours and, by 1965, he was the chair of its Improvement Committee.
He resigned from this following a dispute over the naming of the Queen Elizabeth Bridge. He had instead hoped it would be named for Edward Carson, and believed that this name had been rejected due to party indiscipline.
In 1968, McCullough was elected to the Senate of Northern Ireland. The Senate ceased to meet in 1972, and, although McCullough remained a supporter of the Democratic Unionist Party, he did not stand in any further elections.
On 27 October 2014, he died at the age of 91.