Lyons, who stepped down as chief economist at Standard Chartered earlier this month after 13 years at the bank, will be paid an annual salary of £127,200 for around 30 hours of work per week.
He joins the Mayor’s team of staff at the start of next year.
Johnson said: “It is increasingly evident that if London’s businesses are to stay ahead of the game and compete in the global market, we need the best strategies and policies informed by the very best overarching economic advice.”
Lyons is a well-respected economic commentator in the City, having previously been chief economist DKB International and Swiss Bank Corporation.
He also predicted the UK’s deep recession in 2008 and his team at Standard Chartered came top of a poll of 360 global forecasters by Bloomberg in 2010 and 2011.
“It is a great honour to have been asked by the Mayor to advise him on economic issues and to help contribute to the continued success of London,” Lyons said.