[1] Harrison joined the Parachute Regiment, from the Territorial Army, on 1 September 1989 as a University Candidate on a short service commission.

He was held for twelve days before he managed to escape to a UN base at Kailahun, where 222 Indian troops of the United Nations Mission in Sierra Leone (UNAMSIL) peacekeeping operation were under siege to rebel forces.

Harrison was rescued after about ten weeks in an UKSF/UN assault in which he and more than 200 other UN observers and soldiers were liberated by forces acting on directions from SAS teams and backed by Chinook helicopters.

[3] In 2002 he was appointed a Member of the Order of the British Empire "in recognition of his gallant and distinguished services" while held captive in Sierra Leone.

[4][5][6] On completing a master's degree in Defence Administration at the Joint Staff College, Harrison gained command of a rifle company which in 2003 he led during the invasion of Iraq.

He was appointed to the United Kingdom's Permanent Joint Headquarters, finishing this tour seconded to 3 Commando Brigade in Helmand in Afghanistan.