Ms Symonds, 33, who wore a white dress to exchange vows with the prime minister, is reported to have arrived in a limo about half an hour later.
Musicians were pictured leaving No 10 on Saturday night.
Ms Symonds was first romantically linked to Mr Johnson, 56, by the media in early 2019.
They revealed that they were engaged and that she was pregnant in February 2020.
Their son, Wilfred, was born last April.