Jennings was
commissioned into the
Royal Marines as a
second lieutenant on 1 November 1952. He was made an
acting lieutenant on 20 April 1954, and promoted to lieutenant on 1 May 1956 (with seniority in that rank from 1 February 1956). He was made a
local lieutenant colonel on 29 June 1974, and relinquished the rank on 1 August 1976. He retired from the Royal Marines on 29 November 1976 with the rank of
major. In 1976, Jennings joined the staff of the
House of Commons as a deputy assistant. From 1995 to 1999, he served as the
Serjeant-at-Arms, the parliamentary official responsible for order in the House of Commons. In retirement, Jennings was Chairman of the
St Martin-in-the-Fields Almshouse Charity (2004–2010), the
Bowles Outdoor Centre (2004–2010), and the
English-Speaking Union (2014–2015). ==Honours==