The former England international, 41, joined as an attack coach from Clermont Auvergne in 2013 and helped the club lift the Premiership title in 2014.
Reports linked King with Eddie Jones' England set up last year, but he has left Franklin's Gardens with Saints ninth in the top-flight table.
"Alex has been a valued member of the coaching staff," said CEO Allan Robson.
King added: "Helping Saints win the Premiership in 2014 is one of my career highlights and I wish both the players and staff all the success for the coming season."
Northampton Saints assistant coach Alex King has left the Premiership club by mutual agreement.