Mahon previously had a very prominent role in TV3 soap 'Red Rock' as crooked cop and paedophile Brian McGonigle.
His role in 'EastEnders' will see him play the charming and mysterious Ray Kelly, ex-husband of Mel Owen (Tamzin Outhwaite) and step-dad to her son, Hunter.
Mahon will actually be playing the brother to McCormack's character Ciara - who we seen on the soap earlier in the year as the ex-wife of the Irish gangster Aidan Maguire, played by Patrick Bergin.
Speaking about joining the popular soap, Mahon said: "I remember watching the very first episode of 'EastEnders' back in 1985, little did I think I would be roaming the streets of Walford and hanging out at The Queen Vic all these years later.
"I’m so happy to join such an incredibly hard-working and talented team and I’m very much looking forward to playing out the drama that inevitably ensues on any given day in Albert Square."
"My character, Ray Kelly is not one to tread lightly and his arrival to the Square will cause shock waves for many of the residents… it doesn’t get much better than that."
You can see Sean in his new role in 'EastEnders' later this summer.