The Metropolitan Police said the 52-year-old was a father-of-three, born in Kent but living recently in the Birmingham area, who had worked as an english teacher.
Officers also believe he was using a number of aliases and also had a range of previous criminal convictions – many of them violent.
He had never been convicted of a terrorist offence, however.
Prime Minister Theresa May earlier described him as being on the "periphery" of Islamic Extremism.
Islamic State have claimed responsibility for the attack, although security sources believe Masood was inspired, rather than coordinated, by the death cult.
The man police believe to be responsible for the terrorist attack has now been formally identified as Khalid Masood
Masood was not born a Muslim but converted to Islam. He was said to be a "fitness freak" and bodybuilder.
Scotland Yard said: “The man police believe to be responsible for the terrorist attack in Westminster yesterday, Wednesday, 22 March, has now been formally identified as Khalid Masood.
“Masood, aged 52 (25.12.1964), was born in Kent and detectives believe he was most recently living in the West Midlands. Masood was also known by a number of aliases.
READ MORE: Who is Khalid Masood?
The man believed to be responsible for yesterday's attack has been named
“Masood was not the subject of any current investigations and there was no prior intelligence about his intent to mount a terrorist attack.
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“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.
“His first conviction was in November 1983 for criminal damage and his last conviction was in December 2003 for possession of a knife.”
London Terror Attack: Latest Pictures Thu, March 23, 2017
4 dead and 40 injured in Westminster terror attack
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Conservative MP Tobias Ellwood (centre) helps emergency services attend to a police officer outside the Palace of Westminster, London
Earlier Prime Minister Theresa May told the House of Commons: "It is still believed that this attacker acted alone, and the police have no reason to believe there are imminent further attacks on the public.
"His identity is known to the police and MI5, and when operational considerations allow, he will be publicly identified.
"What I can confirm is that the man was British born and that – some years ago – he was once investigated in relation to concerns about violent extremism. He was a peripheral figure.
"The case is historic – he was not part of the current intelligence picture.
"There was no prior intelligence of his intent – or of the plot. Intensive investigations continue."
Four people, including the attacker and a police officer who tried to stop him, died in the attack
A woman claiming to have been his neighbour in Birmingham said she was shocked by the news.
Iwona Romek said: “I have been so shocked by it all. They were a nice family, very reserved. He was very calm. I saw the photos on the TV and knew it was the man who lived here.
“He had a wife, a young Asian woman, and a small child who went to school.
"He was a nice guy. I used to see him outside doing his garden. Never any trouble."
Khalid Masood being treated by paramedics following yesterday's attack
Armed police during yesterday's deadly attack in Westminster
She described police raids last night as "a bit scary".
Ms Romek said: "I just heard a lot of banging and shouting. They were shouting to open the door. It was all a bit scary. There was a helicopter above and men in white suits.
London Terror Attack Victims Thu, March 23, 2017
ISIS claims responsibility for London attack that killed at least three victims
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Four people died, including the attacker, and a security officer who attempted to stop him, in yesterday's attack.
Kurt Cochran, from Utah, and mother-of-two Aysha Frade also died when they were struck on Westminster Bridge.
This is a breaking story. More to follow…