The 38-year-old actor - who plays ladies man Barney Stinson on TV show' How I Met Your Mother' - announced his engagement to partner David Burtka after gay marriage was made legal in New York on Friday.
He tweeted: "It PASSED! Marriage equality in NY!! Yes!! Progress!! Thank you everyone who worked so hard on this!! A historic night!
"David and I did propose to each other, but over five years ago. We've been wearing engagement rings for ages, waiting for an available date. (sic)"
Neil and David have been in a relationship since 2004 and became parents to twins Gideon Scott and Harper Grace in October 2010 via a surrogate mother.
Neil had been vocal about his desire to marry David, writing on twitter: "Please vote yes on marriage equality. It'd mean a lot to me and my family.
"I'd sure love to get married. Please, NY Senate, vote in favor of marriage equality today. My family would really appreciate it."