The episode showed the pair flipping a Spanish style home, but Haack was in Tennessee with her fiancé, Joshua Hall, and unable to attend their typical end of project walkthrough in person.
The pair figured out the listing price via FaceTime, but the situation got Haack thinking. Haack and El Moussa met up in person when she returned, and had a serious discussion while Hall and Heather Rae Young, El Moussa's wife, waited in their respective cars.
"I have something I want to talk to you about," Haack told El Moussa. "I feel like it's not really fair to you or myself to continue house flipping with you."
"Huh," he replied, "Have you really thought about this?"
"I've thought about it a lot," she replied. "I thought about it more on the last trip to Tennessee and I think it's what's best. I can't be at these houses for you like I should. It's been an amazing run. There's been ups and downs, but it's mostly been fun."
In a confessional, El Moussa admitted that he was "pretty surprised at Christina's decision to leave the flipping business."
"I do wish her the best of luck, but, you know, sometimes in life change can be good for everybody," he added.
Meanwhile, Haack told the cameras that "this was not an easy decision to make and it's definitely bittersweet."
"After 10 years of flipping houses together we have been through it all, but it's time for me to start a new chapter in my life," she added, "and it's definitely the right choice."
Speaking to Haack, El Moussa called it "the end of a legacy," before his ex reminded him that they'd still see each other due to the two kids they share, Taylor, 11, and Brayden, 6.
"That's true," he acknowledged. "We still have two little monsters."
With that, El Moussa and Haack got in their separate cars to greet their respective partners and look forward to a new journey ahead.
Before the finale aired, El Moussa took to Instagram to reflect on his Flip or Flop journey.
"I can’t believe this journey started in May 2010 with a home video and a dream of flipping houses on TV… it’s crazy to think 12 years later it’s coming to an end," El Moussa wrote alongside throwback pics of him and Haack. "I’ve spent almost a third of my life committed to this show."
"It’s almost impossible to capture in a post what this show and experience has meant to me," he added. "After an entire decade of filming Flip or Flop, it truly feels like I have a massive family all around the world. All of you inspire me every day to keep pushing and keep going. From the crew working so hard behind the scenes, to our fans watching us every week, the support has been incredible."
El Moussa went on to thank "everyone who has put their blood, sweat and tears into this show to make it what it is today" including the crew, contractors, HGTV and more.
He next expressed his gratitude for Haack, whom he was married to from 2009 to 2018.
"A special thank you to @christinahaack for taking this journey with me," he wrote. "I think we did a pretty good job and I’m excited for what the future holds for both us and wish her nothing but the best!"
El Moussa concluded his post by assuring his fans he's "not going anywhere," writing, "I can’t wait to show you what comes next!"
For her part, Haack shared a shot of a renovated kitchen, writing that "tonight’s episode is another incredible before and after and I’m really proud of the way it turned out!"
As for the end of an era, Haack wrote, "I want to say thank you again to each and every one of you who has watched throughout the years. I’ll always remember some of the crazy / very gross homes we have walked into, the amazing transformations and everything I learned about house flipping / design along the way."
Like El Moussa, Haack will continue on at HGTV after the end of Flip or Flop.
"The good news is I'll be bringing more stunning projects to Christina On The Coast + more to come," she teased. "To be continued…"
"Tarek El Moussa and Christina Haack are long-time, fan favorite stars on HGTV and it's true that Flip or Flop is coming to an end after an epic 10-season run as a top-rated unscripted series," the network said in a statement at the time. "More than 90 million viewers have watched the popular series since its premiere in 2013. We look forward to seeing more of Tarek and Christina's real life, real estate and renovation adventures in upcoming episodes of their solo series Flipping 101with Tarek El Moussa and Christina on the Coast."
A source told ET at the time, "Christina and Tarek felt ready to move on from Flip or Flop. They both have a lot going on in their own careers and personal lives, and the timing felt right. They have a stable co-parenting relationship, but working together so closely was sometimes a lot."
Shortly thereafter, a source close to Haack told ET that the designer and her ex "communicate for the kids," but said that "off set, they aren’t really friends."
"Christina is looking forward to working on new projects, and their own solo projects. It was time to be done," the source said. "The series is ending with great ratings. She is starting to film Christina on the Coast next week."