Marina Wheeler, 55, shares four of the Prime Minister's six children and applied for a decree absolute in February.
It is understood that this was granted before Ms Symonds gave birth to Mr Johnson's sixth child Wilfred last week.
Ms Wheeler, a leading barrister, started divorce proceedings last year after she and Mr Johnson separated in 2018.
Based on Mr Johnson's estimated cash and assets, Ms Wheeler could receive around £4million from the divorce.
On February 18, she obtained permission from the Central Family Court in London to apply for a decree absolute.
The paperwork was assumed filed “immediately” and the decree was issued.
Mr Johnson and Ms Wheeler had four children during their 27-year-marriage – Lara Lettice, who is 26, Milo Arthur, 24, Cassia Peaches, 22, and Theodore Apollo, 20.
The Prime Minister also has a fifth child, an 11-year-old daughter Stephanie, with art consultant Helen McIntyre.
He was also previously married to socialite Allegra Mostyn-Owen from 1987 to 1993.
