General Sir Nick Carter will continue as Chief of the Defence Staff until the end of November 2021.
The British military chief said it is "a great honour" to be asked to stay on in his post and that there is "much to do".
The 62-year-old had been expected to retire in June.
"Last year’s substantial settlement of £24.1 billion for defence, announced by the Prime Minister in November, gives us the stability and confidence to modernise the Armed Forces to meet the challenges and seize the opportunities that will be laid out in the Integrated Review," Gen Carter said.
Prime Minister Boris Johnson has asked the Chief of the Defence Staff to remain in post to ensure "continuity and stability while the conclusions of the Integrated Review are implemented", the Ministry of Defence said.
The Integrated Review is expected to be published on 16 March, followed by the Defence Command Paper on 22 March.
The selection of a new UK Armed Forces chief will begin in the autumn, the MOD added.
Commenting on the decision to keep Gen Carter in post until late November, defence analyst Professor Michael Clarke said it is "quite important".
"He was due to step down in June, which would have been right in the middle of getting the whole new approach launched properly.
"Staying until probably the end of November, I think, it’s quite an important transition."
Prof Clarke added: "It is very important that we don’t lose continuity and that General Carter stays in post at least for a few transitional months."
There is speculation the Integrated Review could mean cuts to the size of the British Army.
"I think it’s quite likely that the [Integrated] Review will slim down the Army even further, and that would be controversial," the defence analyst said.
"Sir Nick Carter was the man who established the modern nature of the current Army before he became even Chief of the General Staff […] so he’s the man to shepherd through what is likely to be a slimming down of the Army and a transition of the Army to a more high-tech role," Prof Clarke added.
The Chief of the Defence Staff is the professional head of the Armed Forces as well as the principal military adviser to the Prime Minister and the Defence Secretary.
General Sir Nick Carter has been in post since June 2018, taking over from Air Chief Marshal Sir Stuart Peach.
He joined the Army in 1977 and served in Germany, Cyprus, Bosnia, Kosovo, Northern Ireland, Iraq and Afghanistan.
Prior to taking on the role of Chief of the Defence Staff, he was Chief of the General Staff, heading the British Army for four years.
Cover image: General Sir Nick Carter walking towards Number 10 in early 2020 (Picture: PA).