Thorpe has served in the role since April 2014, prior to which he was chairman of both the Racecourse Association and Arena Leisure.
He also served on the boards of the Horserace Betting Levy Board and British Horseracing Authority.
“I have decided that, after nearly a decade as chairman, it is the right time to step down,” Thorpe said. “I look forward to some more time in retirement, as well as the opportunity to pursue other interests.
“It has been a significant source of pride to see the company flourish and grow over the past few years, and without doubt we have assembled an incredibly smart, passionate and hard-working team. I am equally delighted to have been part of creating a new industry structure, a major improvement which should serve the sport well in to the future.
“I am sure the company will only continue to get stronger and more successful over the coming years, and I am proud to have played a part in that foundation for growth.”
ARC chief executive Martin Cruddace added: “David has been an exceptional chairman, and we have been very fortunate to have benefitted from his experience and expertise for such a long tenure.
“He has played a pivotal role in the growth and transformation of the company over that time, and his detailed knowledge of the industry has been an enormous help.
“Both myself and our shareholders would like to thank him for his expertise, hard work and support, and completely understand his decision to step down.”
The announcement comes after ARC in November last year completed the acquisition of omnichannel content distributor Vermantia.
ARC, which held a majority stake in Vermantia since March 2018, said the full acquisition fully aligns with its strategy to invest to build vertically integrated technologies, from content to broadcasting and distribution, creating a “unique” offering in global racing.
Vermantia will continue to develop its product line, enter new markets and expand its presence in existing regions.