By Agence France Presse
Police said the rampage that left four people dead in London was "Islamist-related", and people who knew Masood in recent years described him as religious.
Known by "a number of aliases", including his birth name Adrian Russell Ajao, he had a string of convictions -- although none of them were terror-related.
Born on Christmas Day 1964 into a middle-class family in Kent in southeast England, Masood had been living in the central English city of Birmingham, where armed police have raided several properties since Wednesday's attack.
He was brought up in the town of Rye, on the southern English coast, although his mother and step-father have since moved to Welsh-speaking west Wales, where they rear chickens and run a textile business, according to reports.
Former schoolmates told British newspaper the Daily Mail that he had been a popular student who excelled academically and at sport, and that he was a "happy-go-lucky" character.
Masood lived in several places in southern and central England over the years, working for 12 years at a cleaning chemicals company in southern England, according to The Sun newspaper.
London's Saudi Arabian embassy said Friday that he had worked in the country as an English teacher between 2005 and 2009.
"During his time in Saudi Arabia, Khalid Masood did not appear on the security services' radar and does not have a criminal record in the Kingdom of Saudi Arabia," said an embassy statement.
He had a violent streak: between 1983 and 2003 he racked up convictions for grievous bodily harm, possession of offensive weapons and public order offences, according to police.
British media said he also went by the alias Adrian Elms -- a name he used in 2000 when he was sentenced to two years in jail for slashing a man's face in the Sussex village of Northiam, not far from where he grew up.
Prime Minister Theresa May said that he was once investigated by British domestic spy agency MI5 in relation to "concerns about violent extremism", but was not part of the current "intelligence picture".
- 'Laughing and joking' -
The night before the attack, Masood stayed in a hotel in Brighton on the southern English coast, where he told staff he was visiting friends.
He had rented a car that he later used to mow down pedestrians on Westminster Bridge from a branch of Enterprise in Solihull in Birmingham, the company confirmed.
Sabeur Toumi, manager of the Preston Park Hotel in Brighton, told Sky News that police had shown him a photo to confirm Masood's identity.
The attacker, who had also stayed the previous Friday, had been "very friendly, laughing and joking", and discussed his family in Birmingham, he said.
"It is very shocking because these days you don't know who are the bad ones and who are the good ones. He was just like any other guest who checked into the hotel."
- Religious man -
The Islamic State group claimed that one of its "soldiers" had carried out the London attack following a call to target countries fighting the jihadists in Iraq and Syria.
Police said they were seeking to establish if Masood was inspired by terrorist propaganda, or if "others have encouraged, supported or directed him".
British media described Masood as a Muslim convert, citing neighbours who described him as devout.
"He was a nice guy. I used to see him outside doing his garden," Iwona Romek, a former neighbour, told the Birmingham Mail.
"He had a wife, a young Asian woman and a small child who went to school," she said.
Other media have reported that he was a married father-of-three.
Romek said the family had abruptly moved out of their house in Winson Green, a neighbourhood in western Birmingham, around Christmas without saying goodbye.
More recently Masood may have been living in a flat next to a Persian restaurant and a pizza parlour in the upmarket Edgbaston neighbourhood, according to reports.