There had been speculation over whether Symonds, who has quickly become a mainstay in British tabloid newspapers, would join Johnson at the residence. The PM’s office had told CNN last week it did not know whether the pair would be living together.
The move makes Johnson the first prime minister in living memory to live with an unmarried partner in Number 10.
His divorce from his second wife, Marina Wheeler, with whom he has four children, is yet to be finalized. The pair announced they were splitting after 25 years of marriage last September, shortly after which Johnson’s relationship with Symonds was confirmed.
Interest in the couple was heightened after details of an altercation between the pair at Symonds’ flat were reported by The Guardian last month.
Johnson comfortably won the Conservative Party leadership election last week and was appointed prime minister on Wednesday. Symonds joined his aides outside the door to Number 10 as he made his first speech as leader.
She has also enjoyed a successful political career, working as part of the campaign team that helped to get Johnson re-elected as mayor of London in 2012.
Symonds went on to become the youngest-ever director of communications at the party, at 29, and analysts say she’s been crucial in tidying up Johnson’s famously disheveled persona during the leadership campaign.
CNN’s Sheena McKenzie contributed reporting.