E! Online reports that the actor proposed to his fiancée on Saturday night with an engagement ring that had once belonged to Karstens's grandmother.
The couple plan to tie the knot in Vermont next July.
"I love waking up with my best friend every morning," London said.
Karstens explained that she is excited to marry the Dazed And Confused star.
She remarked: "I feel so lucky to have such an amazing man in my life."
The pair have dated for two years.
