The ring in question looked like a round stone set in platinum and was clearly visible on her left hand.
Allison wowed in a long-sleeve Emilia Wickstead gown, which featured a high neckline and daring keyhole open back. The Connecticut-born actress completed her pretty in pink look with glittering cluster earrings that matched her latest wedding accessory.
She pulled her long brunette locks into an elegant bun and kept her makeup to a minimum, opting for smokey eye shadow to contrast with her bright green eyes.
The beautiful small-screen star — who plays the uptight Marnie Michaels on hit HBO show Girls — got engaged to her boyfriend of three years Ricky Van Veen last month.
The couple began dating three years ago after meeting through mutual friends.
Allison has previously told Glamour that she would "walk away from" her career in order to have children with her then-boyfriend.
"I'd love to be a mum — 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."
According to reports, College Humor co-founder Ricky wanted to follow tradition when it came to popping the question, asking Allison's dad, NBC news anchor Brian Williams, for her hand in marriage before asking his actress girlfriend to marry him.