Scotland Yard said Masood was not the subject of any current investigations and there was “no prior intelligence about his intent to mount a terrorist attack”.
However, he was known to police and has a range of previous convictions for assaults, including GBH, possession of offensive weapons and public order offences.
Masood was born in Kent on Christmas Day in 1964 and detectives believe he was most recently living in the West Midlands.
He was also known by a number of aliases.
Masood’s first conviction was in November 1983 for criminal damage and his last conviction was in December 2003 for possession of a knife.
He has not been convicted for any terrorism offences, the Met said.
Three people were killed when the knife-wielding attacker ploughed a car through pedestrians on Westminster Bridge, before storming the Parliamentary estate. He was shot dead.
Earlier on Thursday Prime Minister Theresa May disclosed the terrorist was British-born and known to police and MI5.
He was once investigated in relation to concerns about violent extremism some years ago but was a “peripheral figure”.
The case is “historic” and the attacker was “not part of the current intelligence picture,” Mrs May added.
As police and intelligence agencies mounted a massive investigation to piece together the killer’s movements in the lead-up to the attack:
• Anti-terror officers arrested eight people, and several addresses were raided overnight in London and Birmingham
• Police activity was reported in several other areas of the country
• Investigators are working on the basis that the attacker acted alone
• Car firm Enterprise confirmed the Hyundai used in the attack was one of its vehicles
• Islamic State claimed the killer was one of its “soldiers”
• A review of security arrangements at Westminster was launched
• Tributes were paid to the victims of the atrocity.
The Prime Minister addressed MPs as they gathered at the usual time inside the Palace of Westminster.
In a defiant message to a packed House of Commons, Mrs May said: “We will never waver in the face of terrorism.”
Paying tribute to Pc Keith Palmer, who died after being stabbed, she said: “He was every inch a hero and his actions will never be forgotten.”
The officer’s family described him as “brave and courageous”, saying his friends and relatives are “shocked and devastated”.
The identities of the terrorist’s victims on Westminster Bridge have emerged, one a US tourist from Utah celebrating his wedding anniversary, the other a “highly regarded and loved” member of college staff.
Kurt Cochran and his wife, Melissa, on the last day of a trip celebrating their 25th anniversary, were visiting her parents, who are serving as Mormon missionaries in London.
Aysha Frade, who worked in administration at independent sixth-form school DLD College London, in Westminster, is understood to have been 43 and married with two daughters.
Meanwhile, Islamic State made its first public pronouncement since the atrocity, claiming in a statement: “The attacker yesterday in front of the British Parliament in London was a soldier of the Islamic State executing the operation in response to calls to target citizens of coalition nations.”
Commentators pointed out the terror group has a record of opportunistically claiming attacks and said it was significant the statement did not appear to claim it had directed the strike.
A minute’s silence was held nationwide at 9.33am, including in the Palace of Westminster and at New Scotland Yard, to commemorate the three innocent people who were killed.
Forty other people were injured in the attack, with 29 treated in hospital, where seven remained in a critical condition on Thursday.
The casualties included 12 Britons, three French children, two Romanians, four South Koreans, two Greeks, and one each from Germany, Poland, Ireland, China, Italy and the United States.
Three police officers were also hurt, two of them seriously.
Westminster Bridge reopened on Thursday, less than 24 hours after the attack.