A source close to the couple, who are now expecting their first child together, told UsWeekly on Monday that they tied the knot in secret.
Kail sparked rumors about secret nuptials when he was spotted out and about with Williams while wearing a band on his wedding finger.
PEOPLE confirmed in late December that Williams and Kail were engaged and expecting their first child together.
A source declined to confirm whether Williams and Kail had got married.
Michelle and Thomas met while working on the FX mini-series Fosse/Vernon, which premiered April a year ago. Williams has been private about her love life since late partner Heath Ledger passed away in 2008.
"It's messy behind the scenes", a source told Us Weekly, adding that Michelle and Thomas "fell hard and fast for each other". The Fosse/Verdon actress was previously married to Phil Everum from sometime in the summer of 2018 to April 2019. Around the same time, the "Hamilton" director split with actress Angela Christian, his wife of 13 years.
As she prepares to welcome her second child into the world, Michelle revealed she is taking a backseat from acting.
In January, Michelle gave her daughter and then-fiance a shoutout at the Screen Actors Guild Awards while accepting the award for best female actor in a Television Movie or Limited Series. She said, "om, I share this with you".
'I have something else I have to go do. "I always say to Matilda, 'Your dad loved me before anybody thought I was talented, or pretty, or had nice clothes'". The director is close friends with Lin-Manuel Miranda and directed his musical In the Heights on Broadway.