He began his civil service career in 1958 at the Ministry of Agriculture, Fisheries and Food, becoming Principal Private Secretary to the minister in 1967.

From 1983 until 1987, Williamson had returned to the United Kingdom to serve as Deputy Secretary and head the European Secretariat in the Cabinet Office.

After leaving Brussels, Williamson was created a life peer on 5 February 1999 with the title Baron Williamson of Horton, of Horton in the County of Somerset,[1] and sat as a crossbencher in the House of Lords.

Lord Williamson of Horton was the convenor of the crossbenchers, a task that involves keeping the other non-aligned members up-to-date with the business of the House.

As well as being appointed a Companion of the Order of the Bath (CB) in 1984[2] and a Knight Grand Cross of the Order of St Michael and St George (GCMG) in 1998,[3] Williamson was awarded the Knight Commander's Cross of the Bundesverdienstkreuz by Germany, the Commander Grand Cross of the Royal Order of the Polar Star by Sweden and made Commander of the Légion d'honneur by France.