Kennedy, 44, joins Alan Gaffney's new-look management team at Vicarage Road after a 12-month spell working with Italian club Calvisano.
The much-travelled New Zealander has also held assistant coaching positions with Canterbury, where he had played as a number eight, Western Province and the Stormers in South Africa before working with Alan Solomons at Ulster and Northampton.
Gaffney said: "Someone with his experience is a great acquisition for Saracens and someone who can take the club forward over the coming years.
"We had a lot of applicants for the job from some very highly qualified people but we wanted to take our time to make the right appointment.
"We needed to make sure that whoever was coming in had a similar philosophy as to how we want to play the game. We needed someone who has experience of northern hemisphere rugby, is going to have the respect of the players and be a good communicator.
"This was not a process that we were going to rush. I'm confident we've definitely found the right person.
"I know of Adrian from his time with Ulster, when I was also working in Ireland. He worked with some very talented players and they all speak highly of him. Adrian also worked with Cobus Visagie in South Africa and I know that Cobus is looking forward to linking up with him once again."
Kennedy had been in the running for the head coach's job at Edinburgh before the Gunners were bought out by a private franchise.
Gaffney, who was appointed director of rugby at the end of last season, has already recruited Richard Graham from Bath to be Saracens' backs and skills coach.