The business entrepreneurs confirmed their relationship in January when they shared an Instagram snap showing them enjoying an idyllic, romantic dinner together in Mexico.
And last night, back on UK soil, they made their first red carpet appearance together at Dr Leah's Cosmetic Skin Clinic event in Chigwell, Essex.
Mark was on hand to support his girlfriend at her glamorous night, attended by >em>TOWIE stars Chloe Lewis and Kate Wright, Amy Childs, Frankie Essex and, of course, her backer and business partner Lord Alan Sugar.
Northern Irish-born Leah won The Apprentice in 2013, receiving a £250,000 investment from Lord Sugar in her cosmetic procedures business. Leah, a qualified former NHS doctor, opened her first clinic in central London, and another one is about to open in Loughton, Essex.
She announced last year plans to launch her own skincare range, telling The Belfast Telegraph at the time: "Lord Sugar has been telling me to get a move on - he wanted me to bring it out right away after winning, but I'm a total perfectionist and I want to the range to be really effective before I give it my stamp of approval."
Australian-born Mark, meanwhile, won The Apprentice in 2014, receiving the £250,000 investment in his digital marketing agency, Climb Online.