The Black Swan actress, 30, rented out one of Ashton's favourite restaurants in Hollywood, La Poubelle, to ring in his 36th birthday.
"It was very small," an insider told Us Weekly. "Ashton has been going there for year and years. It's one of his favourite spots, like home, so it was comfortable and they were well taken care of."
Mila, who gets on well with Ashton's friends and is "part of the crew," asked his pals to help plan the night.
"Ashton didn't want a huge thing but they got a good group together," said the source. "He's certainly changed from how he used to like to party years ago. This was more of a wholesome situation. They had a great time."
The loved-up couple will no doubt have looked picture perfect together, enjoying each other's company surrounded by Ashton's nearest and dearest. Dishes on the menu included T-bone steak, shells and cheese with lobster, quail, braised rib and short rib, according to the publication.
Close friends have previously revealed that an engagement is on the cards for the couple who have been dating for two years.
"They talk about getting married and having kids," said an insider. "An engagement is imminent."
Other reports have also suggested that Ashton popped the question in August 2013 on Mila's birthday, but that the couple couldn't make the news public until Ashton had finalised his divorce from Demi Moore in November of that year.
Mila and Ashton first met on the set of popular sitcom That 70's Show where they played on-screen couple Jackie and Kelso for eight years. While Ashton went on to marry Demi in 2005, Mila dated child star Macaulay Culkin from 2002 to 2010.