"When I joined Horizon in November 2007, I was ranked outside the top 100 players in the world and was seeking a new and energetic management team to help me achieve my career ambitions," the Northern Irishman said in a statement.
"I am proud to say that in the intervening years these ambitions have not only been achieved but have surpassed my highest expectations," McDowell added after finishing 11 strokes behind winner Hunter Mahan at The Barclays tournament on Sunday.
McDowell sealed his only major triumph in 2010 and is hoping to make his fourth consecutive appearance in Europe's Ryder Cup team at Gleneagles next month.
"During my time with Horizon, I have become a major champion, played in three successive Ryder Cups and won multiple tournaments around the world," the 35-year-old said.
"I am at a stage in my life where it feels right to move on to the next phase of my career - both in golf and business."
Rory McIlroy, the world number one and McDowell's Ryder Cup playing partner, is a former member of the Horizon stable.
(Writing by Tony Jimenez; Editing by John O'Brien)
