Hours after results confirmed that Johnson had defeated the long-serving Livingstone in Thursday's election, Johnson marched into City Hall, surrounded by a throng of cameramen and photographers, to sign the declaration of acceptance of office.
Before Johnson's arrival at City Hall, Livingstone came in to clear his desk.
Johnson does not officially become mayor until Monday but was busy talking to advisers and selecting his team.
In a speech, Johnson said he planned to have an extra 50,000 affordable homes built in the capital and to focus on tackling crime.
"I think there's a vital necessity to drive out so-called minor crime and disorder as a way of driving out more serious crime," he said.
Noting that a 15-year-old boy had been stabbed to death early on Saturday morning, the latest in a spate of knife crimes in the capital, Johnson said: "I do think it is time that we led the fightback against this dreadful scourge that is the tip of an iceberg of a problem that we face across London."
"This problem of kids growing up without boundaries, of getting lost in tragic, catastrophic, self-destructive choices is the number one issue that we face in this city," he said.
He also made a plea for unity in the city after a fiercely contested campaign.
"It is vital for a mayor of London to bring people together and to unite London and to serve every community in London impartially," he said.
Johnson is now in charge of an 11 billion pound budget covering public transport, police and fire services in a city of some 7.5 million people.
He is set to have meetings with key figures in the capital, including Metropolitan Police Commissioner Sir Ian Blair.
Luxury carmaker Porsche quickly asked Johnson to honour one of his election pledges to scrap a tax on gas-guzzling cars proposed by Livingstone.
In a drive to cut carbon dioxide emissions and flight global warming, Livingstone had wanted to raise the eight-pound daily levy on all cars driving in the city centre to 25 pounds for those with high fuel consumption.
Porsche had called the plan unfair and disproportionate and filed an application for a judicial review in the High Court.
Over the next four years Johnson will also oversee preparations for the 2012 Olympic Games and be responsible for promoting policies on housing, the environment and the economy in Europe's biggest financial centre.
A top official of employers' group, the Confederation of British Industry, Nigel Bourne, said Johnson faced three key challenges; to tackle congestion and ensure the transport network works; raise the capital's skill levels and make the most of the London 2012 Olympic Games.
"We look forward to working with the mayor to find and develop effective answers to London's needs and to make it the best city in the world," Bourne said in a statement.
