Reilly will depart the network at the end of June, but a successor has not been named yet.
Kevin Reilly is stepping down as the head of entertainment programming at FOX in the wake of ratings decline for its shows. The top executive who has been at the network since 2007 announced the decision on Thursday, May 29.
"21st Century Fox is a great company that has provided me with choice seats at the head table of pop culture over two very rewarding stints both at FBC and FX, and I am grateful to have benefited from the leadership of Peter Rice, Chase Carey and Rupert Murdoch," he said in a statement.
"Peter and I have been discussing this transition for a while, and now with a robust new slate of programming for next season and strength in the FBC ranks, it felt like the timing was as right as it could be. I couldn't be more thankful to my team -- a group of creative, tireless and fun people whose fellowship I will miss."
Reilly, who had more than two years remaining on his contract, will leave at the end of June. A search for his successor has begun, with a source saying outside candidates as well as executives who currently work within the company being considered. In the interim period, Reilly's senior executives will report to Peter Rice, chairman and CEO of Fox Networks Group.
Rice said, "Kevin's undeniable creative gift and passion for talent have left an indelible mark on our company, and we're extremely grateful to him for his leadership, beginning with 'The Shield' and 'Nip/Tuck' at FX and continuing through 'Glee', 'New Girl', 'The Following', 'Sleepy Hollow' and 'Brooklyn Nine-Nine' at FOX."
He added, "Kevin and I have been discussing his desire for a new challenge for some time, so while everyone at the network will miss his enthusiasm for adventurous television, we all respect his decision and agreed that after the upfront presentation was the right time. I'm personally extremely grateful to Kevin for his partnership during the past five years and look forward to working with him again in the future."
Reilly's exit comes on the heel of FOX losing over NBC in the 18-49 demo during 2013-2014 primetime TV season. For nearly his entire run, FOX managed to end the season at No. 1 in the key demo, mainly relying on "American Idol". However, the singing competition series dropped double-digits in the ratings in its last season.