Military

After Sandhurst, Marriott was commissioned as a regular officer in May 1977,^[4] joining the 17th/21st Lancers. During 1982 he was a company commander in the Multinational Force and Observers in Sinai, returning to his regiment as Adjutant later that year. In 1985 he was posted as an assistant to the Vice Chief of the Defence Staff (Personnel & Logistics), in 1987 returned to the 17th/21st Lancers as a squadron leader, then in 1990 attended the Canadian Staff College in Toronto. He was an instructor at the Staff College from 1991, before service with the Queen's Royal Lancers in 1993 and taking up an appointment at the British Army Training Unit Suffield in 1995. This was followed by a brief posting to SHAPE in 1998. Later that year he became Commanding Officer of the Queen's Royal Lancers, after which he was Chief of Staff, HQ 1 (UK) Armoured Division from 2000. He took the Higher Command and Staff Course in 2004, then commanded 7th Armoured Brigade from 2005 to 2007, when he was appointed as Assistant Chief of Staff (Operations) at Permanent Joint Headquarters. He was Commandant of the Royal Military Academy Sandhurst from 2009 to 2012.^[3]

While serving in post-invasion Iraq as commander of the 7th Armoured Brigade, Marriott was also in command of all British troops in southern Iraq from 2005 to 2006. While many soldiers were complaining about the politicians who had sent them into action without enough men or equipment, Marriott took the view that the British could never have gained the trust of the people of Basra, thanks to their history. In an interview with the journalist Oliver Poole he said "The people were immersed in cruelty. There's a sad culture of mistrust, especially of authority."^[5] In celebration of his Brigade's nickname, "the Desert Rats", Marriott kept a large silver rat on his desk in Basra.^[6]

In 2012 he was appointed as Colonel of the Queen's Royal Lancers, retiring on 2 May 2015, when the regiment amalgamated with the 9th/12th Royal Lancers (Prince of Wales's) to form the Royal Lancers.^[7]

He was appointed Honorary Colonel of the 7th Battalion The Regiment of Scotland Army Reserve on 2 December 2024.^[8]

Ecclesiastical

Marriott served as a member of the House of Laity of the General Synod of the Church of England from 2005 to 2012,^[9] and was a director of the British Forces Foundation from 2009 to 2012.^[3][10]

Historian

Marriott authored a history of the Battle of Littleferry 1746 which was published in 2022.^[11]