Lavender replaced Richard Gould when he joined Somerset in 2011.
"Being the CEO at Somerset has been an immense privilege and enormously enjoyable," Lavender said.
"We have a very busy season ahead and I am determined to make sure that this is a great one for the club in all respects."
Somerset chief executive Guy Lavender is to leave the county at the end of the season to take up the same role with the MCC.