The happy couple met guests from six hospices on Monday to pay tribute to their "hard work, dedication and compassion" at their first official public appearance together in Number 10.
Pictures from the event show Mr Johnson smiling with guests wearing t-shirts emblazoned with the Noah's Ark Children's Hospice logo, as Ms Symonds laughs and greets visitors.
In a tweet, Number 10 said: "PM @BorisJohnson hosted a reception at Downing Street to thank hospice staff for their hard work, dedication and compassion.
— UK Prime Minister (@10DowningStreet)August 12, 2019
"The Prime Minister was joined by six hospices, where he shared with them how grateful he was for the difficult but vital and caring work they do."
Ms Symonds tweeted it was "such a privilege to meet so many inspiring people who do such incredible work".
A spokeswoman for Number 10 said: "It was a reception that the Prime Minister hosted to thank hospital staff for all their hard work."
Earlier this year, there were concerns over the pair's relationship following reports the police had been called after neighbours' concerns over an alleged argument between Mr Johnson and Ms Symonds.
The couple's appearance came the day after Mr Johnson's estranged wife Marina Wheeler revealed she had been diagnosed with cervical cancer three months ago.
Ms Wheeler, a QC who became Mr Johnson's second wife in 1993 and separated from him in 2018, told the Sunday Times she underwent surgery twice after being given the news in May.
Mr Johnson and Ms Symonds, who live in a flat above Number 11 Downing Street, are the first unmarried couple to live together in Downing Street.
Ms Symonds made her first appearance outside the Prime Minister's residence during Mr Johnson's first address to the nation.