The wedding took place in a “small ceremony” on Saturday afternoon, a Downing Street spokesman said.
The spokesman explained that the couple would celebrate again with family and friends next summer.
Johnson is the first prime minister to get married while in office in nearly 200 years.
The Mail on Sunday reported that 30 guests were invited to the ceremony at short notice – the maximum number of attendees allowed under Covid restrictions in England.
The last prime minister to get married while in office was Robert Banks Jenkinson in 1822.
Symonds was first romantically linked to fifty-six-year-old Johnson by the media in early 2019.
Symonds used to work in the Conservative Party press office, which she joined in 2010, and two years later she worked on Johnson’s successful campaign to be re-elected as mayor in London.
She became the party’s head of communications before leaving in 2018 to work in public relations for marine conservation organisation Oceana.