Born in Nairobi, Colony of Kenya the son of Gerald and Elspeth Carter, Carter was educated at Winchester College and the Royal Military Academy Sandhurst. He was commissioned into the Royal Green Jackets as second lieutenant on 8 April 1978, initially holding a short service commission. Promoted to lieutenant on 8 April 1980, he switched to a full career commission in 1982, and was promoted to captain on 8 October 1984. As a junior officer he served in Northern Ireland, Cyprus, Germany and Great Britain. Promoted to major on 30 September 1991, he attended Staff College, Camberley later that year before becoming a company commander with 3rd Battalion, Royal Green Jackets in 1992. He became military assistant to the Chief of the General Staff in 1994 and, having been appointed a Member of the Order of the British Empire in the 1996 New Year Honours, he joined the directing staff at the Staff College later that year.
Carter was promoted lieutenant colonel on 30 June 1996. In 1998 he was appointed Commanding Officer of 2nd Battalion, Royal Green Jackets in which role he was deployed to Bosnia in 1998 and Kosovo in 1999. For his service in Bosnia, he was awarded the Queen's Commendation for Valuable Service on 7 May 1999. In Kosovo, Carter commanded a group of peacekeepers on a bridge over the River Ibar at Kosovska Mitrovica where he was tasked with keeping apart thousands of Serbs and Albanians gathered on either side of the bridge. Carter later described the role as being the "meat in the sandwich". He was advanced to Officer of the Order of the British Empire on 3 November 2000.
Carter was promoted to colonel on 31 December 2000 (with seniority from 30 June) and advanced to Commander of the Order of the British Empire on 29 April 2003, following service in the War in Afghanistan. He was promoted brigadier on 31 December 2003 (with seniority from 30 June), and in 2004 he was given command of 20th Armoured Brigade, commanding British forces in Basra, at one point stating that British forces could be in Iraq for "as long as a decade". On 7 September 2004 he was awarded a further Queen's Commendation for Valuable Service for his service in Iraq.
Carter became Director of Army Resources and Plans at the Ministry of Defence in 2006 and was given the honorary appointment of Deputy Colonel of The Rifles on 1 February 2007 (the successor regiment to the Royal Green Jackets) – a post he held until 1 November 2009. Promoted to major general on 23 January 2009, became General Officer Commanding 6th Division which was deployed to Afghanistan with Carter as Commander ISAF Regional Command South. In September 2009, referring to the efforts of UK and NATO forces, Carter said that "time was not on our side". After returning to the UK in November 2010, he gave an interview in which he warned that "the insurgency is resilient, and alive and well".
Carter became Director-General Land Warfare early in 2011 and, having been awarded the Distinguished Service Order in March 2011, he was promoted to lieutenant general and appointed Commander Field Army in November 2011 (the role redesignated Deputy Commander Land Forces in January 2012). He was the main architect of the Army 2020 concept and reported on his recommendations in April 2012. He assumed the post of Deputy Commander, International Security Assistance Force (ISAF), under the command of American general, John R. Allen, in September 2012 and, having handed over his command at ISAF in July 2013, he became Commander Land Forces in November 2013.
Carter was appointed Knight Commander of the Order of the Bath in the 2014 New Year Honours. On 21 February 2014 it was announced that Carter would assume the post of Chief of the General Staff. He took up his post and was promoted to full general on 5 September 2014. As of 2015, Carter was paid a salary of between £170,000 and £174,999 by the department, making him one of the 328 most highly paid people in the British public sector at that time. Carter was awarded the US Legion of Merit on 18 March 2016 for services in Afghanistan.
On 1 February 2013, he succeeded Sir Nick Parker as Colonel-Commandant of The Rifles.
Carter has been criticised on several occasions by American officers for his conduct and command while in Afghanistan. He was described by Colonel Harry Tunnell, former Brigade Commander of 5/2 Stryker Brigade Combat Team, as displaying a "gross lack of concern for subordinates" throughout his command in Afghanistan in 2009 and 2010. Further criticism came from Lieutenant General Daniel P. Bolger, who claimed that "young riflemen paid the price" for Carter's "risk-averse" mentality and his unwillingness to allow his troops to defend themselves. Bolger also claimed that Carter refused to visit the front line and only visited safe positions by helicopter, while frequently refusing requests for aircraft and artillery support from troops under his command. Bolger further stated, "He's not the type of general I would put in charge of anything."
These allegations have been vigorously contested by other US officers and British colleagues. For instance, Lieutenant General Ben Hodges who currently commands US Army Europe, and worked under Carter in Afghanistan, wrote to The Sunday Times stating that he was "appalled" by Bolger's representation of Carter. The letter, jointly signed with another former subordinate of Carter, went on to observe that Carter’s emphasis on avoiding civilian casualties had been crucial to mission success, that the soldiers under his command were never denied the right to defend themselves, and that Carter frequently exposed himself to personal danger by deploying forward to the key towns and villages at the heart of the counter-insurgency effort. Carter Malkasian, an author and CNA analyst writing for The Washington Post, said Bolger was "on thin ice" when criticizing those, like Carter, who "tried to protect innocents".
Conversely, Tunnell's judgment whilst under Carter's command was the subject of criticism. A formal US Army review of the conduct of Tunnell's brigade in Afghanistan pointed to its "lack of discipline" and "contempt for the normal Army rules". The report's author concluded: "Colonel Tunnell is no longer in command... If still in command, I would recommend that Colonel Tunnell be relieved of his responsibilities as a brigade commander."
In 1984 Carter married Louise Anne Ewart; they have three sons and one daughter. His interests include golf, cricket, field sports and cycling.