"I think at some point, you've got to step aside and let young guys that are qualified have their shot," Nix said during a hastily called news conference shortly after Bills completed a voluntary minicamp practice. "I never put a timetable on it. I always felt like I'd know when it was the right time.
"And I think it's the right time."
The 73-year-old Nix will not be leaving the team entirely. He'll instead move into a newly created role as a special assistant.
Nix steps down two weeks after overseeing his fourth draft with the Bills, which the team opened by selecting its quarterback of the future in Florida State's EJ Manuel with the 16th pick. And his departure comes with Buffalo in the midst of yet another start-from-scratch overhaul under new coach Doug Marrone.
"I feel strongly that the team is on the right course for success," Nix said. "I think the thing that probably I'm the most proud of will be the next two or three years. I think we've got a really good young roster. I love our head coach. I think he's a guy who's going to win a lot of games here for a long time."
Assistant GM Doug Whaley, entering his fourth season in Buffalo, is expected to take over. Though the Bills did not immediately announce Nix's replacement, the team has spent much of the past year grooming Whaley to succeed Nix.
In signing Whaley to a long-term contract extension in February, both Nix and team president Russ Brandon referred to Whaley as someone who will "transition" into the top job "when the time comes."
Whaley is a former scout and executive with the Steelers. He was heavily involved in scouting this past year, and also involved in the Bills coaching search in January.
On Monday, Brandon declined to say whether Whaley will take over. Brandon would only say that a succession plan is in place, but refused to provide a timetable as to when a new GM would be appointed.
The next general manager will become the team's fifth since John Butler was fired during the 2000 season. The lack of continuity has been blamed on the team's struggles.
Buffalo has not enjoyed a winning season since going 9-7 in 2004. And it has not made the playoffs in 13 seasons — the NFL's longest active drought
Nix's departure is not regarded as a surprise. He had been sidestepping questions about his future in Buffalo for much of the past nine months.
On Monday, Nix said he didn't make up his mind until recently.
"I already had a flight and was scheduled to go to St. Pete to start on the 2014 draft," Nix said, referring to the team's annual scouting meeting in St. Petersburg, Fla. "And to be honest with you, my energy level's good as it's ever been, but I wasn't all excited about that trip."
Nix also intends to spend more time with his family, starting with attending his grandson's 9-year-old birthday party this weekend.
Nix has 50-plus years of football experience as a coach, scout and executive at both the college and NFL levels. After serving as a Bills scout in the 1990s, he left Buffalo to spend the next seven seasons in the San Diego Chargers front office.
Buffalo lured him out of retirement to work as a scout in 2009. The Bills then promoted Nix to the GM's job later that year. He took over for Brandon, who stepped down as general manager to devote more time to the team's business operations.
Nix has had mixed results, with the team going 16-32 since he took over. That included consecutive 6-10 finishes the past two seasons, which led to coach Chan Gailey being fired on Dec. 31.
Gailey's firing was a setback for Nix. Aside from hiring Gailey, Nix had voiced his support for the coach through much of last season.
Nix's vision to rebuild through the draft has taken longer than he expected. Aside from Manuel, he's used first-round picks to select running back C.J. Spiller, defensive tackle Marcell Dareus and cornerback Stephon Gilmore.
And Nix's one major foray into free agency has yet to pay dividends.
The Bills defensive struggles continued last season despite the high-priced addition of defensive end Mario Williams, who was signed to a six-year $100 million contract in March 2012.
"I knew the first two (seasons) were going to be struggles. But we thought we had things in place to win more games (last season)," Nix said. "But we just didn't get it done for whatever reason."
Before drafting Manuel last month, Nix had failed to address the team's most important need at quarterback.
Nix put his faith in Ryan Fitzpatrick by signing the journeyman to a long-term contract extension after the Bills got off to a surprising 4-2 start in October 2011. Fitzpatrick failed to play to the expectations and was cut this off-season.