Mamie Gummer and Ben Walker: Engaged!

Actress Mamie Gummer and boyfriend Ben Walker are engaged!

"He proposed two days ago," Gummer, 26, announced Monday in New York at the Whitney Museum of American Arts Gala, noting that he "surprised" her with a custom-made ring. "He did it on one knee!"

Mamie is the daughter of Meryl Streep.

Dating for about a year and a half, Gummer and Walker met working on a short film, and later performed together in Broadway's Les Liasions Dangereuses.

"My mom loves him!" the Taking Woodstock actress gushed.

Mamie Gummer is the second eldest child to multiple Oscar-winner Streep, 60, and her sculptor husband Don Gummer, who have four children together.

"We are very happy!" Gummer said of Walker.