And recently, the American Idol alum shared the key to making her relationship with the legendary producer work.
"Therapy. A good therapist," Katharine exclusively told E! News' Francesca Amiker while promoting her Radiance by Absolute™ x Katharine McPhee jewelry line. "Or that you just have someone in your life or another couple in your life that is helpful when there's discord, disagreements."
The Smash star—who tied the knot with David in 2019—continued, "I really feel like you can talk through most everything, but you have to have the language, the vocabulary to do that. So that takes practice and self-awareness and having skills to facilitate those conversations."
The two aren't just collaborators in life as they also frequently take the stage and release music together, including their recent Christmas Songs album. And Katharine—who first met David in 2006 when he served as a guest mentor on American Idol before reconnecting with him over a decade later—said there's an "ease" that comes with collaborating on their work.
"We met each other 18 years ago in this exact space—like a recording studio, in a work environment," the 39-year-old noted. "So, that's truly where we know each other the best is knowing the ease, and the greatness, and the way he works and just always gets the best thing out of people."
In 2021, David, 73, and Katharine welcomed their first child together: son Rennie, 2, who joins the composer's adult daughters Allison, Amy, Sara, Erin and Jordan from his previous relationships. As for what the toddler is up to these days, the "Over It" singer says he's "talking up a storm."
It looks like he's inherited his parents' musicality too and has already started dabbling with the drums.
"I would say that the drumming thing has been a continuous theme in his life since he was 14 months, but he's hit another level in his drumming that is really impressive," Katharine explained. "People have been more impressed than I think we have been. We're like, 'Oh yeah, he really loves it. It's good. We don't know if it'll be a real thing.' But now I'm going, 'Hmmm, this might be a real thing.'"
But just because he has musical parents doesn't mean they expect Rennie to follow suit.
"I would love for him to follow in his dad's footsteps and just be really well-versed in all different types of musical instruments," Katharine continued. "Obviously, he's 2 and a half. We're not pushing him. We're just letting him do his thing. But, I would love for him to have a working understanding of all instruments a lot like his dad. I think he's on his way—that's for sure."
For now, she's just focusing on enjoying life's everyday moments with their little one.
"I just love everyday mundane things like picking up your kid from school," she added. "To me, that is so fun because he says, 'You're back! You're back!' He's so happy that I'm there, and that's not gonna be forever that I get to do that."
And she's staying plenty busy. In addition to her family and music career, Katharine is launching her new jewelry collection Radiance by Absolute™ x Katharine McPhee on HSN Oct. 25, with live appearances at 7 p.m. and 8 p.m. ET.
"It's inspired by things that I love—by pieces that I love, pieces that I think are wearable on an everyday basis but also can be sort of zhuzhed up for more of a fancy evening," the artist, who is also performing some of her Christmas Songs hits on the network two days later, said. "So we tried to sort of hit everything in our very first round. Hopefully, there will be many rounds to go."
To look back at Katharine and David's love story, keep reading.