The 24-year-old has pledged his future to the Shrimpers until the summer of 2011 despite rejecting an earlier contract offer.
McCormack found the net nine times in 51 appearances last season.
"I'm pleased that everything has been sorted out. I know it was put out that I rejected the club's first offer of a contract but I was never looking to get away," he said.
"We enjoyed a great end to last season and, if we can carry on that way, then I really do think promotion is a possibility next year."