Herjavec, 53, proposed to Johnson, 39, after meeting on Dancing with the Stars in Spring 2015 and dating for only 10 months.
"I never thought a year ago I'd meet someone who would change my life completely!" Johnson, who did not compete on DWTS' most recent 21st season, captioned an Instagram photo of the happy couple on Sunday. "I'm the luckiest girl in the world because last night the love of my life @robert_herjavec asked me to marry him and I said yes!"
"I still can't believe that a year after walking into a dance studio of all places, my life has changed this much," Herjavec, who popped the question with a beautiful 6.5-carat ring, toldPeople. "Kym is an incredible woman and I'm just so excited for what the future brings for us."
Johnson and the Shark Tank star reportedly partied after their engagement at home with over 30 friends in attendance -- including Johnson's mother, Carson Kressley and her DWTS friends.
"To celebrate tonight with our family and friends, and my mom who flew in from Australia, wow -- I'm just overwhelmed and so happy," Johnson told the magazine. "He's a wonderful man and I'm so lucky."
The couple started out as friends, but according to Johnson, their connection quickly grew as they gained more respect and admiration for each other.
The businessman didn't even own up to their romance when host Tom Bergeron let the cat out of the bag by saying Herjavec didn't win the mirrorball trophy at the time of their Week 8 elimination but he appeared to win Johnson's heart.
After their stint on the show, Herjavec and the pro dancer escaped on a romantic getaway to Palm Springs, CA.
Herjavec announced last April he and ex-wife Diane Plese, with whom he has three kids, were legally separating after 15 years of marriage. This will be the first marriage for Johnson, who recently launched a dieting book and fitness DVD.