Jeff Zgonina, who had previously served as the Commanders' assistant defensive line coach during the past two seasons, will be promoted in Mills' absence.
"Ron Rivera said he believed the DL room needed a change," Garafolo tweeted.
Mills had served as Washington's defensive line coach during each of the past two seasons.
The Commanders, led by Pro Bowl defensive end Chase Young, enter the 2022 NFL season ranked No. 2 among all NFL defensive line groups, according to Ben Linsey of Pro Football Focus, despite having the sixth-highest average of yards allowed per play in 2021, with both Young and Montez Sweat missing a combined 15 games last season.
"Expectations were sky-high for Washington’s defensive line and defense overall entering last season," Linsey wrote. "And there were few more disappointing performances than Washington’s defense ending the 2021 campaign ranked 27th in yards allowed per play (5.7)."
Washington finished third in the NFC East last season, marking the fourth time the franchise has missed the playoffs during the past five seasons.
Mills' reported termination comes just three days after his father, former All-Pro linebacker Sam Mills Jr., was posthumously inducted into the Pro Football Hall of Fame.
The Commanders will host the Carolina Panthers in their preseason debut at FedEx Field on Saturday (August 13).