The actor, who played Rhys Ashworth in the soap from 2005 to 2014, shared a photo of Mark getting down on one knee on Instagram.
Mark proposed to his fiancée while on a romantic trip in Greece aboard a yacht.
Andy wrote, “I shouted YAS!!” and added a diamond ring emoji to the photo.
He paired it with a photo of him wearing a sailor cap while the couple relaxed on the yacht while drinking beer.
Bronagh Waugh, who portrayed Cheryl Brady in Andy’s Hollyoaks, agreed with them and shared more photographs on her social media account.
‘A tremendous HONOUR to be on the yacht and share the moment when he said YES!!’ she wrote.
‘It was the most BEAUTIFUL and EXTRAVAGANT day… It’s quite appropriate for a prince who’s met his Prince.
‘Love you both so much @andymossss @markbenn85 #beyoncé #helikeditsoheputaringonit,’ alongside images of the proposal moment and the day on the yacht