Fitzsimmons was also an active member of the Ulster Unionist Party and was the President of Duncairn Unionist Association. He was elected to the Parliament of Northern Ireland at a by-election in 1956, representing Belfast Duncairn. In 1961, he was made Assistant Parliamentary Secretary to the Ministry of Finance, an assistant whip position. He also became Parliamentary Secretary to the Ministry of Commerce. He was then rotated through a series of Parliamentary Secretary positions, serving with the Ministry of Home Affairs from 1963 to 1964, the Ministry of Health and Local Government from 1964 to 1966 and also the Ministry of Development for a period in 1965.[1]
In 1968, Fitzsimmons' daughter married a Roman Catholic. As a result, he resigned from the Orange Order.[2] At the 1969, former independent Unionist Member of Parliament Norman Porter stood against Fitzsimmons as a candidate whose opposition to Catholicism was in no doubt, but Fitzsimmons held his seat.[3]