Millard has overseen the men’s and women’s teams during his time in charge, with both winning their respective league titles in 2021.
The men’s team reached the European Champions League semi-finals for the first time last season, losing to eventual winners Toulouse.
Quins are currently fourth in the Premiership table with six games of the season remaining.
“My time at Harlequins has been nothing short of amazing, and I cannot thank the board, staff, players, and supporters enough for creating so many memories and friendships for my family and myself,” Millard added.
“I will continue to support everyone at the club and drive all areas forward until the end of the season.”
