The festivities kicked off at home, where El Moussa woke up to gold balloons celebrating his big day.
"Happy birthday to my baby," Young wrote alongside a clip of the birthday boy standing in front of the balloons.
The real estate agent went on to share a shot of herself, El Moussa, and his two children, Taylor, 10, and Brayden, 6, posing around the balloons. El Moussa shares his kids with his ex-wife, Christina Haack.
After their morning at home, the engaged couple continued the birthday celebrations by spending the day on a boat with a private chef.
Following their boat ride, Young posted a selfie with her fiancé, writing, "In glam now getting ready for the birthday dinner, but I just love him so much. So relaxing on the water today."
The birthday celebrations culminated with a party at Twenty Eight Restaurant and Bar in Irvine, California, where Young surprised El Moussa "with a dinner with all our family and closest friends."
Young went on to share pics from the soiree, which included a lavish meal, 40th birthday decorations, and custom cookies. "Had to do it big for his 40th," Young wrote alongside one pic.
When the couple arrived at the party, Young wrote that El Moussa was "so surprised" by the gathering.
On his Instagram Story, El Moussa shared a shot with his fiancée, writing that he was "so lucky."
"Thank you baby," the HGTV star added.
El Moussa additionally gushed over Young in a post on Instagram, sharing a pic of them smiling at the birthday bash.
"This is 40. ♥️ Blessed doesn’t even begin to describe it. Thank you to my love @heatherraeyoung for making my day extra special by surprising me all day long and thank you to everyone for the birthday wishes," he wrote. "Cheers to 40 and cheers to many many more great years."
Young also posted about her fiancé's birthday on Instagram, sharing a lengthy tribute to her "soulmate," along with photos from their time together.
"Happy birthday to the man who puts a smile on my face every day," Young wrote. "To the man who walked into my life on July 4th two years ago and made me believe in love again. And to the man who I get to spend forever with."
"I’m tearing up as I write this -- you are the most special man and words don’t do it justice," she continued. "The most hardworking, loyal, and direct man and the best daddy. Always looking out, protecting, and caring for the ones you love and making me feel like I’m the only person in the room since the day we met."
Young wrote that, while she "always believed in soulmates growing up," she "never really knew what it meant" until she met El Moussa.
"You’re my best friend and I knew from our first date, as you were sitting there nervously sweating and obsessively putting on chapstick, that you were my person," she wrote. "I love the life we’ve created together, I love our little family, and nothing is going to make me happier than becoming Mrs. El Moussa."
"We talk about growing old together all the time but no measure of time will be long enough," Young concluded. "Happy birthday baby, @therealtarekelmoussa. I love you ♥️"