The former member of the Grey's Anatomy cast, who now stars on Private Practice, turned 40 on Saturday and says she had no angst about the milestone.
"Probably, honestly, I would have cared more if I were still single," the star tells Marie Claire.
"But I've also been working through all the big things that you grapple with as a female. I feel I'm in such a place of peace in my life that 40 feels like a great marker of a great year."
Kate Walsh says she had let the idea of marriage go - until she met Alex Young, 36, a co-president of film production at 20th Century Fox.
"I met him, and I knew unequivocally," she says. "Is that strange? I used to want to hit people who said, 'You know when you know' about someone. It made me want to throw up. But I'm the annoying one now."
Kate Walsh and Alex Young moved in together just a month later, got engaged in May, and made it official when they got married Labor Day weekend.
BIRTHDAY GIRL: Kate Walsh was all smiles Saturday, her big 4-0.
The beautiful Private Practice star describes her new husband as "super intelligent, super smart, very handsome" and "a true gentleman."
"It's so easy to be cynical, but it's not how he lives," she says. "I've always known I've needed someone who lived from his heart."