The Giants are 2–10 after a 24–17 loss to the Oakland Raiders on Sunday.
General manager Jerry Reese was also relieved of his duties as well, NFL.com reports.
Defensive coordinator Steve Spagnuolo will be the interim head coach.
McAdoo and Reese's firings comes less than a week after the team decided to bench starting quarterback Eli Manning. Manning had started 210 straight regular season games before giving way to backup Geno Smith.
Last month, ESPN reported that several Giants players said McAdoo has lost some respect in the locker room and players were giving up on the season.
McAdoo received some backlash for suspending cornerbacks Dominique Rodgers-Cromartie and Janoris Jenkins in recent weeks.
The Giants were blown out 51–17 at home against the Los Angeles Rams in Week 9 and co-owner John Mara told NJ.com that the loss spoke for itself.
McAdoo told reporters that he believes the Giants can make a run. That sentiment was all but destroyed the very next week when New York lost to the previously winless San Francisco 49ers.
McAdoo succeeded Tom Coughlin as the Giants' head coach after serving two years as the team's offensive coordinator. In 2016, he went 11–5 to tie Dan Reeves' franchise record for most regular season wins by a first-year head coach.
He led the Giants to their first playoff appearance since 2011 but lost to the Green Bay Packers in the NFC Wild Card Game.