In a surprise announcement Johnson, who had been seemingly cast into the political wilderness after the June 23 referendum, was handed a task that will inevitably be dominated by handling Britain's departure from the European Union following last month's vote to leave, AFP reported.
Johnson had been widely expected to stand for the leadership of the governing Conservative Party after "Remain" campaign leader David Cameron announced his resignation as prime minister in the wake of the referendum.
Johnson had been the bookmakers' favorite to become prime minister.
However, his running mate Michael Gove decided to stand for the job himself and Johnson swiftly pulled out in a shock statement which had been expected to announce his candidacy.
It is the first role in government for Johnson, who was London's mayor from 2008 to 2016, a spell which included overseeing the 2012 Olympic Games.
The appointment amounts to a remarkable comeback for the mop-haired 52-year-old, whose career was thought to have crashed and burned after what was seen as a political assassination by Gove.
Johnson was the second appointment in May's new government after she shifted Philip Hammond from the Foreign Office brief to become Britain's new finance minister.
"The queen has been pleased to approve the appointment of Boris Johnson MP as Secretary of State for Foreign and Commonwealth Affairs," Downing Street said in a statement.
On leaving the prime minister's Downing Street office, Johnson immediately got into a ministerial car and went to speak to staff at the Foreign Office.