Johnson played under head coach Bill Belichick as a player with the New York Giants and the Cleveland Browns, and joined his staff at the Patriots in 2000, but was passed up for the position of defensive coordinator in 2012 in favour of Matt Patricia.
"I could never have asked for a better coach/boss/friend in those 26 years," Johnson told ESPN. "Thanks for always supporting me and I know Bill will continue to support me no matter what.
"Thank you New England Patriot fans; I will miss you the most."
Johnson has five Super Bowl rings, three of which he earned as part of the Patriots coaching staff.