Mackay will end his three-year stint at The Hive at the end of the month.
"It was difficult decision personally but the right one in the interests of the club," Mackay told the club's official website. "In some ways the biggest achievement has come in the last six months. We saved the club from administration and closure, and found a new home so that the Broncos could survive.
"When I look back, I can honestly say that I have thoroughly enjoyed my time at the Broncos. It's a great club which shows amazing character and determination along with a never-say-die attitude from staff, players and the fans alike."
Mackay is the former chief executive of Sussex County Cricket.