Mr Johnson, 56, exchanged vows with Ms Symonds, 33, in front of a handful of close friends and family, according to the Mail on Sunday,
He is the first prime minister to marry in office since Lord Liverpool married Mary Chester in 1822.
It comes just six days after the couple sent out save-the-date invites suggesting guests keep Saturday, July 30 free for their wedding .
When asked if the Prime Minister had wed one guest said: “You are not barking up the wrong tree. The couple will be making an announcement shortly.”
One No 10 aide said: “I have been strictly told not to comment.”
The couple’s 30 guests were invited at the last minute – with even Tory aides unware that the nuptials were going ahead.
The Sun reports that the church was cleared shortly after 1.30pm, with staff telling visitors it was being closed due to lockdown.
At about 2pm, a limo carrying the bride arrived. Classical music was said to be played as they walked down the aisle and exchanged vows.
Ms Symonds wore a white dress – but no veil.
The ceremony was carried out by Father Daniel Humphreys, who baptised their one-year-old son Wilfred last year.
Armed police were said to be standing by for the ceremony.
One witness said the cathedral went into lockdown for the half-hour service. They told The Sun: “It was closed for about half an hour and they all came out after.
“It’s not very often we have weddings here, and when they came out they were all bungled into a car.”
Mr Johnson and Ms Symonds were the first unmarried couple to live in Downing Street. They live in the four-bedroom residence at No11 Downing Street rather than the smaller flat in No 10.
The couple got engaged in December 2019. Mr Johnson proposed while the pair were on holiday in Mustique. The proposal reportedly came following his win in the December General Election.
It is Mr Johnson’s third wedding – and Ms Symonds’ first.
He wed Allegra Mostyn-Owen in 1987, but that marriage was annulled in 1993 — the same year he married Marina Wheeler.
The couple had four children but separated in 2018.
Their final divorce came weeks before the news of Mr Johnson’s engagement to Ms Symonds.
Weddings in England are currently subject to strict coronavirus restrictions.
Ceremonies and receptions are permitted for up to 30 people in Covid-secure venues.
Dancing is advised against due to the increased risk of transmission, apart from the couple’s traditional first dance.
Northern Ireland’s First Minister Arlene Foster tweeted: “Huge congratulations to Boris Johnson & Carrie Symonds on your wedding today.”
The wedding comes at the end of a difficult week for the Prime Minister in which his former aide Dominic Cummings branded him unfit for office.
The Prime Minister’s former aide said Ms Symonds had been desperate to oust him from his role, and had sought to put her own friends in key positions.
Mr Cummings also claimed that in February 2020, when the pandemic was becoming a major global crisis, Mr Johnson was “distracted by finalising his divorce.
