Chief executive Geoffrey King said: "We are extremely sorry to see Garry leave the club. He has done a fantastic job in the development of our commercial activities with some notable achievements leaving the Club with a great platform to take advantage of the new stadium. We wish him well with his future career and thak him for his efforts."
The role will be filled by Ian Riddoch, who will join the Shrimpers in February.
Ian has a CV that includes being Head of Sales and Marketing at Aberdenn Football Club for several years as well as the top marketing job at Wigan Warriors RLFC and CEO of Glasgow Warriors RUFC.
King continued: "We are delighted with the appointment of Ian who has the necessary skills and experience to continue where Garry left off helping to continue the off pitch development of the Club. We are looking forward to working with him."