Offerman married actress Megan Mullally on September 20, 2003. They met while acting in The Berlin Circle, a play produced by the Evidence Room Theatre Company, and married after dating for 18 months. While they were still dating, Offerman appeared in a guest role on Mullally's sitcom, Will & Grace, in a 2001 Thanksgiving episode. He later guest-starred in a different role in a 2018 episode. Mullally also had a recurring role on her husband's sitcom, Parks and Recreation as Ron Swanson's second former wife, Tammy 2. The couple have also appeared together in films such as The Kings of Summer and Smashed, as well as performing voice work together in Hotel Transylvania 2, Bob's Burgers, and The Great North.
Offerman and Mullally did a live comedy tour in 2016, the theme of which was their sex life.Offerman and Mullally launched a podcast in 2019, In Bed with Nick and Megan, discussing their personal lives and interviewing guests.On whether he is a libertarian like his character Ron Swanson, Offerman has stated, "While I admire the philosophy of the libertarian mindset, I think it's proven to be ineffectual in actual governance. So no, I'm not. I'm a free-thinking American." Upon voting in the 2020 Democratic Party presidential primaries in his home state of California, Offerman tweeted a photo of himself with an "I Voted" sticker, adding the caption "#VoteWarren", referring to the Democratic Senator from Massachusetts and then-presidential candidate, Elizabeth Warren. In June 2020, he tweeted his support for Charles Booker, a progressive Democratic candidate in the U.S. Senate Democratic primary in Kentucky.