The couple first met while starring on the 1966 comedy "The One and Only, Genuine, Original Family Band."
She was 21, and he was 16 at the time, and Hawn recalled Russell being "adorable" but "much too young" for her to date.
Hawn and Russell have worked together numerous times through the years, and claim their relationship is so strong because they haven't walked down the aisle.
"We have done just perfectly without marrying. I already feel devoted and isn't that what marriage is supposed to do? So as long as my emotional state is in a state of devotion, honesty, caring, and loving, then we're fine," she said in an interview with Now To Love.
"We have raised our children brilliantly; they are beautiful people. We did a great job there, and we didn't have to get married to do that. I like waking up every day and seeing that he is there and knowing that I have a choice. There is really no reason to marry."
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It's not that they haven't experienced marriage before. Russell was married to "Elvis" actress Season Hubley for four years before their divorce was finalized in 1983. The former couple has one son, Boston.
Hawn, 77, married director Gus Trikonis in 1969, but their relationship ended in divorce in 1976.
She went on to marry musician Bill Hudson for six years before their marriage ended in divorce in 1982. Hawn and Hudson have two children together; Kate Hudson and Oliver Hudson.
"For people like us, the marriage certificate wasn’t going to create anything that otherwise we wouldn’t have," Russell told People of their relationship. "I don’t know – 40 years isn’t enough to finally say, ‘Well I guess…’"
"Our children got married … Oliver’s very happily married and Wyatt’s very happily married," Hawn added. "Katie got married once and that didn’t work, and she’s with this most amazing human and I don’t know if she’ll marry again. The point is that we all tried marriage and sometimes marriage doesn’t work.
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"It’s not about the marriage. It’s about the people and the relationship, and the will to stay together. And that’s a big one because if you want it, you can have it. You’ve got to give things up, but the joy and the excitement of being together and touching the toes of somebody at night is really a nice feeling."
Hudson has followed her mom's non-traditional outlook on relationships, with the "Almost Famous" actress recently opening up about her own blended family.
The 43-year-old actress shares son Ryder, 18, with Chris Robinson, son Bingham, 11, with ex-fiancé Matt Bellamy, and daughter Rani Rose, 4, with her fiancé Danny Fujikawa.
"It might not look traditional from the outside, but on the inside I feel like we’re killing it," Hudson told The Sunday Times in November. "The unit that I’ve created with three children with three different fathers is a seriously strong unit, and it’s ours."
She added, "I’m not interested in forcing some conventional idea of love or marriage. I’d like to be able to grow intimately with my partner for a long time, but I also don’t have rose-coloured glasses on."
"My goal in life is that I want to feel love and I want to give love, but I’m also practical, so, one day at a time," Hudson noted. "I work really hard at relationships because I like them."
"My parents have done an amazing job of continuing that dance," the Academy Award nominee noted of Russell and Hawn.
The "How To Lose a Guy in 10 Days" star married rocker Robinson when she was just 21-years-old.
During an appearance on "The World's First Podcast," the "Fool's Gold" star, explained why she decided to get married at such a young age.
"I just jump into the deep end of everything I do," Hudson told hosts Erin and Sarah Foster. "I'm in love with him. I'm not gonna pretend like, 'Oh, we should wait.' I'm madly in love, and I want to marry him, and so I just didn't think twice. I'm kind of still like that, except with a little more wisdom under my belt."
Hawn has also explained more about why she and Russell never made their union legal during an appearance on "Loose Women" in 2015.
"I would have been long divorced if I'd been married," Hawn said. "Marriage is an interesting psychological thing. If you need to feel bound to someone, then it's important to be married."
Hawn continued, "If you have independence, if you have enough money and sense of independence, and you like your independence, there’s something psychological about not being married because it gives you the freedom to make decisions one way or the other.
"For me, I chose to stay. Kurt chose to stay, and we like the choice."
When Hawn and Russell received his-and-hers stars on the Hollywood Walk of Fame in 2017, the legendary actors could not help but show affection for each other.
"It turned out to be a lovefest," Hawn told People. "The last thing he said was, 'I’ll always cherish you.' I didn’t expect him to say those things. I had no idea he was going to be so beautiful."
On Valentine's Day 2022, the Academy Award-winning actress honored her man with a heartfelt post.
"Love is a Many-Splendored thing. Happy Anniversary Loverboy," she wrote.