Over the past five years the 37-year-old former England fly half has built up a reputation as one of the most exciting coaches in European rugby. King joins the Saints from Clermont Auvergne, where as assistant coach he helped the French club win the Top 14 for the very first time in 2010, and reaching the 2012 semi-final of the Heineken Cup.
This season Clermont remain undefeated in a Heineken Cup pool which includes Leinster and Exeter and currently lie second in the Top 14 table.
But while King says that he is focused on bringing more silverware to the Stade Marcel Michelin this season he is looking forward to developing himself further as part of Jim Mallinder's coaching staff at Franklin's Gardens.
"I'm very excited about the prospect of joining Northampton Saints, which is one of the biggest clubs in Europe," King commented. "In Jim Mallinder the Saints have a director of rugby that I respect greatly and a good, young team with plenty of talented English players. The club also has a great support base and I hope that I'll be able to help bring success to Franklin's Gardens.
"I've had an amazing six years in France as a player and a coach. I always wanted to play in France and immerse myself in a different culture. I have done that and learned a great deal by working with great coaches like Vern Cotter, Joe Schmidt and Franck Azema.
"I feel that I am more knowledgeable about the sport and hopefully I can bring that to Northampton. I'm certainly looking forward to working with Jim, Dorian West and the other coaches at the Saints and believe that it is the right fit for me professionally and personally. It's the right time for me to come home to England and to a club like the Saints, which has a very bright future under Jim's leadership."
As a player King won everything there is to win in England, including two Heineken Cups and four league titles with London Wasps, and Mallinder says that this knowledge, along with his more recent experience in France, will help the Saints take the next step in the club's development.
"We have been inundated with applications from coaches over the past few weeks, but I believe that Alex is the right person to help us move forward as a group," the Saints director of rugby commented. "As a player he was part of an excellent London Wasps team, and as a coach he has helped Clermont Auvergne become one of the top clubs in Europe.
"He has built up a considerable amount of experience in a short space of time and is a coach who has some exciting ideas. I'm looking forward to Alex joining us in the summer and establishing himself at Franklin's Gardens."
The Saints also confirmed that Alan Dickens will be remaining at Franklin's Gardens in a combined role that will see him continue as Academy Manager and in the first team coaching group.
"Alan is an important member of our coaching group and I am looking forward to him continuing his excellent work leading the Saints Academy while developing himself as a coach with the first team," Mallinder added.