According to E!, the proposal went down at the Chicago Water Tower in Illinois, as Christmas carolers sang in the background.
Shortly after the news broke, ET caught up with Mortimer’s co-star, Luann de Lesseps, who had one word about the couple’s news -- “finally!”
“I'm glad for her. You know, it's been a long time coming,” added Lesseps, who was promoting her Countess and Friends cabaret tour, kicking off on Nov. 30. “God knows we've been back and forth about Scott, and all that. And, I'm happy for her, you know?”
“I hope it works out, because they've had their ups and downs,” she continued. “I hope this one is going to stick and it's here to stay.”
Like fans of the show, Lesseps found out about the engagement through the news, but was hardly surprised.
“She didn't [text the news,] but I knew it was inevitable,” she said. “I saw her at Watch What Happens Live at BravoCon, and she looked really happy and it's finally happening! I'm happy for her.”
As for what Mortimer’s nuptials will mean for her future on RHONY -- given that Kluth is based in Chicago and the show revolves around New York -- Lesseps was unsure about how things might pan out.
“We'll see where that goes,” she said. “One makes a choice in life, and I don't know where she's going to land. I wish her the best and hope that she has a long, romantic, wedded blissful life.”
Mortimer’s engagement comes after the pair got back together, having gone their separate ways in 2018.
At the time, Mortimer told Andy Cohen on Watch What Happens Live With Andy Cohen that it wasn’t the first time they had split.
“It's so hard to be in a relationship, particularly when we don't live in the same town. He's in Chicago, so busy with CouponCabin," she said.
It seems they successfully mended their relationship and prior to the proposal. The couple spent a fun weekend in Chicago, with Mortimer posting a video on her Instagram account, showing them out clubbing.