McCallum was in charge of all counter-terrorism investigations and risk management in the run-up to, and during, the London Olympics
Britain on Monday named Ken McCallum, a career spy who led the response to the attempted murder of former Russian double agent Sergei Skripal, as the new head of its MI5 domestic intelligence agency.
McCallum, who has worked in MI5 for 25 years, spent his first decade in MI5 focusing on Northern Ireland but then began battling Islamist militants, developing cyber security and ensuring the safety of the 2012 London Olympics.
McCallum was in charge of all counter-terrorism investigations and risk management in the run-up to, and during, the London Olympics, MI5 said.
Then, after former Russian double agent Skripal and his daughter Yulia were found poisoned with a Soviet-developed nerve agent in 2018, he led the agency's response to the attempted murder.
"In 2018, Ken took charge of the MI5 response to the attempted assassination of Sergei Skripal, within the unprecedented UK-led international response from intelligence allies," MI5 said.
Britain blamed Russia for the attack in which a British citizen died. Russia denied any role, though Western powers expelled dozens of Russian spies working under diplomatic cover.
MI5 said one of McCallum's top priorities was enabling MI5 to seize the fast-moving opportunities provided by technology, including machine learning. He enjoys mountain hiking.
He succeeds Andrew Parker who retires in April.
MI5, established in 1909 to counter German espionage ahead of World War One, is tasked with protecting British national security. Its main job is currently countering international terrorism though it is also a counter-intelligence agency.
"MI5's purpose is hugely motivating," McCallum said in a statement.
"Our people - with our partners - strive to keep the country safe, and they always want to go the extra mile. Having devoted my working life to that team effort, it is a huge privilege now to be asked to lead it as Director General."