Lewer was Forest Green's six manager in the past six years and was only appointed in September 2004 after the sacking of Tim Harris after Forest Green's poor start to the season.
Lewer only had a contract until the end of the season, and had been given the task of keeping the club up, which proved to difficult in the end.
Club chairman Trevor Horsley told the club's official website: "We've agreed to terminate Alan's contract with immediate effect, so this was his last game at the club, I'm not saying we made a mistake in appointing him, but for various reasons it hasn't worked out."