The"little service" took place on Saturday afternoon, a Downing Street spokesman said.
The spokesman added that the couple will observe again with family and friends .
The stealth affair, clarified by PA Media as a"secret wedding," was reportedly held in front of close family and friends, according to a number of British newspaper accounts.
"Congratulations @BorisJohnson and @carriesymonds on your marriage now," the UK Work and Pensions Secretary Therese Coffey wrote in a Twitter post.
Children's Minister Vicky Ford also tweeted a congratulatory message.
"Many congratulations to @BorisJohnson and @carriesymonds & large appreciate to Wilf. So many weddings have been delayed and bothered by covid.
On April 29, 2020, Symonds gave birth to the couple's first child, Wilfred. In precisely the exact same month that the Prime Minister was hospitalized with coronavirus.
Later in 2020, she made her first joint television appearance with Johnson to thank NHS workers for treating him in hospital and delivering their baby.
The union comes just days after the couple were said to have sent save-the-date cards to relatives and friends for an event on July 30, 2022.
Both announced they were also engaged to be married in February 2020.
He was also formerly married to Allegra Mostyn-Owen for six years.
Johnson could marry in a Roman Catholic church because his first two marriages were not conducted under Catholic ceremonies, Matt Chinery, an ecclesiastical and canon lawyer said on Sunday. Some had asked how a twice-divorcee was allowed to get married in a Catholic Church.
Talking to Times Radio, Chinery stated that if you are a Roman Catholic, then there is a requirement that you must have your wedding managed by a priest or deacon and any wedding which is not conducted that way is"not valid" unless there was"previous permission" from your bishop to marry outside a Catholic Church.
"In case you are a Catholic, a baptized Catholic and your first two weddings were not in a Catholic Church dominated by a Catholic minister therefore the Roman Catholic Church does not recognize those two unions as valid," Chinery clarified with regard to Johnson.
Symonds, 33, has enjoyed a successful political career, functioning as part of the campaign team that helped to get Johnson re-elected as mayor of London in 2012.
At age 29, Symonds became the youngest-ever director of communications in the Conservative Party. Critics say she had been crucial in tidying up Johnson's famously disheveled character through the leadership campaign.