Thalassitis signed a professional two-year contract with Stevenage in May 2011, having joined the club's Centre of Excellence ahead of the 2010–11 season, just after Stevenage's promotion to the Football League. He made his first-team debut in August 2011. Two months later, Thalassitis joined Conference South side Boreham Wood on loan, returning to his parent club in January 2012. He was loaned out once again a month later, signing for Hayes & Yeading United on a one-month loan deal. During the 2012–13 season, Thalassitis was briefly loaned out to Cambridge United in September 2012, before joining Bishop's Stortford on loan two months later. He then spent the remainder of the season on loan at Boreham Wood, his second loan spell at the club. Ahead of the 2013–14 season, Thalassitis joined Ebbsfleet United on a season-long loan. In 2017, Thalassitis appeared on the ITV2 reality series Love Island. He retired from football in December that year.