The Prime Minister was joined by three girls who won a competition to choose designs for the PM's Christmas card.
A children’s choir also performed Silent Night at the ceremony outside Number 10 on Wednesday evening.
Mrs May said: "These three girls each designed one of my Christmas cards I'm sending out to thousands of people.
She then addressed the youngsters, telling them, "it’s very good that you're all here with me today", before the lights were switched on after a countdown from five.
The 18ft 6in tree, adorned with baubles and fairy lights, was delivered from Gower Fresh Christmas Trees in Three Crosses, Wales.
The occasion gave Mrs May some respite as the country was plunged into a growing Brexit crisis on Wednesday.
The Democratic Unionist Party has flatly refused to sign off a deal with Ireland, with Mrs May failing to make a breakthrough in telephone talks with DUP leader Arlene Foster.
Earlier Mrs Foster had warned she would “not be rushed” into an agreement on the Irish border issue.