Prior to his election to the European Parliament, Nicholson had been the Member of Parliament (MP) for Newry and Armagh from 1983 until his defeat in the by-elections of 1986, when he and others resigned and stood again to protest against the Anglo-Irish Agreement; Nicholson lost his seat to Seamus Mallon of the Social Democratic and Labour Party (SDLP), the only seat to be lost.

[3] Along with all other unionist MPs, he resigned from the House of Commons in December 1985 as part of a wider protest against the Anglo-Irish Agreement of that year and to secure a renewed mandate from their electors.

He contested the seat again at the 1987 general election[3] but demographics in the area had shifted against unionism; nationalist and republican candidates have held it ever since.

He was one of five Quaestors in the European Parliament, becoming the first ever MEP from Northern Ireland to hold such a senior position when elected on 21 July 2004; he was re-elected in 2007.

[6] Nicholson describes himself as a "euro-realist"; he is opposed to the creation of a federal Europe and is against the adoption of the euro as the currency of the United Kingdom.

[3] His son, Sam Nicholson, was a councillor and deputy mayor for ABC Borough Council, and a candidate for the 2017, 2019 and 2024 general elections in Northern Ireland for the constituency of Newry and Armagh.