A former president of the National Farmers' Union, Plumb was first elected to the European Parliament in 1979. In 1982 he was appointed as Leader of the Conservative Delegation, and elected as Chairman of the European Democratic Group. Following the elections of 1984, Plumb was able to negotiate a deal with the leaders of the European People's Party (EPP) Group, resulting in his own election as President of the European Parliament in 1987, making him the only British MEP to have ever held this post. Upon assuming the presidency, Plumb stood down as leader of the Conservative MEPs and group chairman, and was elevated to the House of Lords as Baron Plumb.

Plumb remained an MEP after the end of his presidential term, and in 1994 again assumed a leadership position in the Conservative Delegation (see below).