While the 25-year-old plays struggling single Marnie Michaels in award-winning series Girls, it seems her love life couldn't be further away from her character's.
The happy news was confirmed by Allison's rep to People magazine on Wednesday.
CLICK ON PHOTOS FOR FULL GALLERY
The daughter of NBC news anchor Brian Williams met her now fiancé through mutual friends and the couple have been dating for three years.
She told Glamour earlier this month that she would "walk away from" her career in order to have children with her then-boyfriend.
"I'd love to be a mom – and not have to bring my kids into my trailer … Or a balance [of both]," Allison said at the time. “But you’ve [got to] call it on the fly to a certain extent."
Allison with her fiancé Ricky Van Veen, pictured in the middle
According to reports, College Humor co-founder Ricky wanted to follow tradition when it came to popping the question, asking Allison's dad for her hand in marriage before asking his actress girlfriend to marry him.
Allison has risen to fame with her role on acclaimed series Girls, created by co-star Lena Dunham.
The pair, joined by fellow star Zosia Mamet, hit the red carpet at the premiere of the third season of the show at Cineworld Haymarket in London mid-Januray. Allison dazzled in an Alexander McQueen dress and clutch, Brian Atwood shoes, and Jessica McCormack jewels for the UK premiere.