Walker was commissioned into the
Irish Guards as a
second lieutenant (on probation) on 9 September 1990 as part of his undergraduate cadetship. In September 1993, his commission was confirmed and he was promoted to
lieutenant back-dated to 11 August 1992. He began active service in the Irish Guards in 1993, and saw operational tours in Northern Ireland and Iraq. He was promoted to
captain on 11 August 1995. In 1997, he joined
22 Special Air Service Regiment. He was promoted to
major on 30 September 2000. He attended the
Advanced Command and Staff Course from 2001 to 2003. Following his time with Special Forces, Walker transferred from the Irish Guards to the
Grenadier Guards on 11 November 2008. He served as
commanding officer of the 1st Battalion, Grenadier Guards from 2008 to 2010. With the Grenadiers, he served a tour in Afghanistan, during which the
Ridgeback PPV in which he was travelling was blown up by an
improvised explosive device (IED): all six soldiers inside escaped uninjured, but the vehicle was thrown into the air, had its wheels blown off and its armour shredded. He was promoted to
colonel on 30 June 2012. Walker commanded the
12th Armoured Infantry Brigade from 2013 to 2015, He has since held staff appointments at
Army Headquarters and the
Ministry of Defence. and appointed
Director Special Forces. He was promoted to
lieutenant general and became
Deputy Chief of Defence Staff (Military Strategy and Operations) on 16 April 2021. He was appointed a
Knight Commander of the Order of the Bath (KCB) in the
2023 New Year Honours, thereby granted the title
sir. Walker served as
Colonel Commandant of the
Royal Army Veterinary Corps from 1 March 2017 to 1 June 2022, and Regimental Lieutenant Colonel of the Grenadier Guards from 18 June 2017 to 18 June 2022; both ceremonial appointments. He was Honorary Colonel of the Cambridge University Officers' Training Corps until June 2024. He was appointed Colonel Commandant of the
Honourable Artillery Company on 30 May 2024.
Chief of the General Staff It was announced in December 2023 that Walker had been selected to be the next
Chief of the General Staff, the professional head of the British Army, and would take up the post in June 2024. He took up the post on 16 June 2024. On the same day, he was promoted to
general and appointed
Aide-de-camp general to the King. On 23 July 2024, Walker warned of the threat posed by
Russia,
China,
Iran and
North Korea. He said the UK must be prepared for a possible
war with Russia by 2027. Walker said there were "no winners" in Russia's
invasion of Ukraine, adding that "it is an utter devastation for both sides and lost generations." He said that with the current way of fighting, it would take Russia five years to control the four regions of Donetsk, Luhansk, Kherson and Zaporizhia that
Russia claims as its own, and it would cost Russia more than 1.5 million
casualties. He said: "It doesn’t matter how it ends. I think Russia will emerge from it probably weaker objectively – or absolutely – but still very, very dangerous and wanting some form of retribution for what we have done to help Ukraine." In 2025, he was considered for the appointment of
Chief of the Defence Staff, the professional head of the British Armed Forces, but came runner up to
Sir Rich Knighton. ==Personal life==