He was educated at Marlborough College from 1976 to 1981 and went on to read Arabic and Islamic Studies at Pembroke College, Cambridge.
Lorimer was commissioned into the Parachute Regiment on 11 December 1982. He was promoted to lieutenant on 11 December 1984, to captain on 11 December 1988 and to major on 30 September 1994. After serving in Northern Ireland, he was promoted to lieutenant-colonel on 30 June 1999 and to colonel on 30 June 2003.
Lorimer was appointed Commander of 12th Mechanized Brigade in June 2005 and was deployed to Iraq and then Afghanistan.
He was promoted to brigadier on 31 December 2005. Promoted to major general on 23 November 2010, Lorimer was appointed as the Chief of the Defence Staff"s Strategic Communications Officer.
He was appointed General Officer Commanding 3rd (United Kingdom) Mechanised Division on 6 June 2011. Promoted to lieutenant general on 9 July 2013, he was appointed Deputy Commander, International Security Assistance Force (ISAF).
Lorimer handed over his responsibilities of Deputy ISAF commander to Lieutenant General Carsten Jacobson in June 2014 and became Chief of Joint Operations in January 2015.
Lorimer was appointed a Member of the Order of the British Empire (Administration Member of the Order of the British Empire) in the 1997 Birthday Honours.