The 35-year-old has signed a three-and-a-half year contract with Ipswich after five years at Old Trafford.
McKenna moved to Manchester United as the club's under-18 coach in 2016 before making the step up to join the coaching set ups of Jose Mourinho, Ole Gunnar Solskjaer and latterly Ralph Rangnick.
"It has been a great honour to work for Manchester United for the last five years. When I arrived as Under-18s lead coach in 2016, I had the privilege to work within a world-leading youth development system, coaching excellent talent in a truly fantastic environment," the 35-year-old Northern Irishman said via United's website.
"I'll always be grateful to Jose Mourinho and Ole Gunnar Solskjaer. I have learnt so much during the past four years and it was a pleasure to be part of such a dedicated group of staff. I would like to thank Ralf Rangnick, I have really enjoyed the short time I have worked with him and I really appreciate his understanding throughout this process.
"From the start of my coaching career, it has always been my ultimate ambition to move into football management and I now have a fantastic opportunity to do that at Ipswich Town, an amazing club with a fantastic history.
"I want to thank the staff and players that I have worked with over the years and, of course, wish Ralf and this incredible club all the best for the future."
Ipswich are currently 12th on the League One standings.
"I'd like to thank Mark Ashton and the owners for putting their faith in me and Martyn to take the club forward," Kieran told Ipswich's website.
"Leaving a club like Manchester United was obviously a difficult decision, but I strongly believe in the opportunity to build something here.
"It feels like the right time, project and club to make my first step into first-team management.
"For now, the focus for the team has to be on a huge game on Saturday. After that, I cannot wait to meet everyone and get to work."
