Mr Johnson’s office confirmed reports in the Mail on Sunday and The Sun that the couple wed at the Roman Catholic Westminster Cathedral in front of a small group of friends and family.
“The Prime Minister and Ms Symonds were married yesterday afternoon in a small ceremony at Westminster Cathedral,” Downing Street said.
“The couple will celebrate their wedding with family and friends next summer.”
The newlyweds have reportedly sent save-the-date cards to family and friends for a celebration on July 30, 2022.
Under current coronavirus restrictions in England, no more than 30 people can attend a wedding.
Mr Johnson, 56, and 33-year-old Ms Symonds, an environmental advocate, announced their engagement in February 2020 and have a son together, 1-year-old Wilfred.
The marriage is Ms Symonds’ first and Mr Johnson’s third. He has at least five other children from previous relationships.
The last British prime minister to marry in office was Lord Liverpool in 1822.
On Friday, an ethics inquiry found the Prime Minister acted “unwisely” in renovating his Downing Street apartment without knowing where the money had come from but it cleared him of misconduct.
A member of the opposition Labour Party, Jon Trickett, suggested the wedding was “a good way to bury this week’s bad news” on Mr Cummings’ testimony and the report on the funding of the Downing Street flat.