But up until now there has been no sign that the couple had tied the knot.
Recently, though, Martin piqued interest in his relationship status when he made this comment about it.
“Let’s leave that a mystery,” he said on the subject of his marital status. “What I like about our relationship is that we choose it. I’m not saying we’re not married, though.”
Martin, 42, is currently in Canada filming the new TV series Fargo (based on the 1996 Oscar-winning film), and admitted to only having been home twice since last October.
“It’s a very heavy price,” he said of the time apart from Amanda and their two children, eight-year-old Joe and Grace, five.
“My main priority in any job is when is the soonest I can get back to the three people I love most in the world. I even ummed and ahhed over The Hobbit.”
But although he may not be at their Hertfordshire home that much, Martin still believes that his family would describe him as “grumpy”, reports the Daily Express.
“If you ask my children and Amanda, they will definitely say I am pretty grumpy and hard to live with sometimes,” he admitted. “I also know that I can be playful and full of joy.”
Martin also described himself as a hands-on father, and added: “I enjoy reading and telling my children spooky stories. I’m quite a disciplinarian: I can be a shouter. But I can be a very demonstrative kisser and hugger.”
However, some lessons learned in fatherhood certainly surprised him and tested his patience at times.
“It goes without saying that you’re going to love your kids, but what you’re not expecting is wanting to kill everybody in your house. I’m fortunate in that Amanda is generally a slightly nicer person than I am. If it were purely up to me, my kids would probably be vegetarian Catholic Marxists.”
But instead, his children love nothing more than sitting down and watching The Hobbit films – in which Martin plays Bilbo Baggins.
“I thought the spiders would really do Grace in, but she wasn’t scared by the peril and the violence,” he explained. “The thing that most upset her was the bit where the dwarves come round and basically eat Bilbo out of house and home. Grace is inconsolable at the idea that they have stolen all Daddy’s food – she thinks I’m being bullied.”
Fargo, also starring Colin Hanks, Billy Bob Thornton and Greta Grimly, is due to start on Channel 4 on Sunday, April 20