Known by "a number of aliases", London's Metropolitan Police said he had been convicted for a string of offences but none of them terror-related.
Born on Christmas Day 1964 in Kent in southeast England, Masood had been living in the West Midlands where armed police have staged several raids since the attack, storming properties in the city of Birmingham.
Over the course of two decades, Masood chalked up a range of convictions for assault, grievous bodily harm, possession of offensive weapons and public order offences, police said, with the offences taking place between 1983 and 2003.
But Masood had never been convicted of terrorism offences and "was not the subject of any investigations," the police said, noting there was "no prior intelligence about his intent to mount a terrorist attack".
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At 52, his age has been highlighted by commentators as unusual, with most Islamist extremists behind similar attacks far younger.
Prime Minister Theresa May said he was once investigated by the intelligence service MI5 "in relation to concerns about violent extremism".
Although the police believe Masood acted alone, the Islamic State group claimed he was one of its "soldiers" acting on a call to target countries fighting the jihadists in Iraq and Syria.
Police declare 'terrorist incident'
'A nice guy'
Masood rented the car used in the attack from the Solihull branch of Enterprise, on the outskirts of Birmingham, the company confirmed in a statement.
According to the BBC, he told the car rental company that he was a teacher.
"He was a nice guy. I used to see him outside doing his garden," Iwona Romek, a former neighbour of his told the Birmingham Mail.
"He had a wife, a young Asian woman and a small child who went to school," she added, pointing out that the family had abruptly moved out of their house in Winson Green, a neighbourhood in western Birmingham, around Christmas.
Other media have reported that he was a married father-of-three.
