The Patriots said Johnson, 32, told the team of his decision a day before veterans were scheduled to begin training camp. His retirement makes the linebacker position for the Super Bowl champions much weaker, since Tedy Bruschi decided to sit out the 2005-06 season after suffering a minor stroke during the off-season.
"It is with deep regret that I have decided to retire from football," Johnson said in a statement issued by the team. "The decision was not an easy one, but life sometimes has a timetable all its own. I can no longer ignore the severe short- and long-term complications of the concussive head injuries I have sustained over the years."
Johnson was a second round pick by New England in the 1995 NFL Draft out of Colorado. The 6-foot-4 inch, 253-pounder played in 136 regular season games and had 865 tackles and 11½ sacks.
2014: The Year in Music [PHOTOS]