A person with knowledge of the decision told The Associated Press that McIlroy will create his own team run by his father, Gerry, along with close friends and associates. The person spoke on condition of anonymity because McIlroy hasn't announced the change.
Graeme McDowell, a close friend on tour who also is represented by Horizon, said the split was "fairly amicable" and that McIlroy wanted to surround himself with family.
"That's fair enough," McDowell said Friday at the World Match Play Championship. "But it's disappointing, as I am disappointed to lose him as a management stablemate. But we will go forward, still very good friends."
McIlroy denied speculation last week at The Players Championship about leaving Horizon. He was accompanied by Sean O'Flaherty of Horizon, when he left Sunday after tying for eighth place.
"We cannot make any comment," Conor Ridge at Horizon Sports said in an email.
McIlroy, who started the year in a slump and still hasn't won, is to play next week in the BMW PGA Championship at Wentworth.
The Irish Independent first reported that McIlroy was leaving.
"Management is a funny thing, and when things are maybe not going 100 per cent on the golf course, it's natural to question everything you are doing from relationships, business and just everything you do," McDowell said. "Sometimes we decide to make choices and decisions, and take new paths that we perhaps feel like for the bigger picture will help us improve and get better.
"Rory has made a decision about his management structure," he said. "I don't know the reasons because I haven't seen much of him the last few months."
McIlroy appears to be following a model set up by Masters champion Adam Scott, who set up his own company headed by his father, Phil, with a small group around him.
The change was a surprise, much like when McIlroy left Chubby Chandler of International Sports Management in 2011, just four months after the 24-year-old from Northern Ireland won the U.S. Open by eight shots with a tournament record 16-under 268.
He knew Ridge and the Horizon staff through his close friendship with McDowell, a fellow Ulsterman and U.S. Open champion. McIlroy last year won the PGA Championship by a record eight shots among his five wins worldwide, which made him No. 1 in the world and allowed him to win the money title on the PGA Tour and European Tour.
Horizon negotiated a mega deal with Nike Golf, which is said to be worth upward of $20 million a year. Horizon also brought sponsorship deals with Bose and Omega, structured in such a way that McIlroy's apparel and golf bag were not splashed with logos. With Nike and Omega, he was able to end his previous contracts with Titleist and Audemars Piguet. He has Santander Bank on his bag, a deal that began under Chandler.
"Business-wise, Rory is in the best shape he's ever been in his life, and while he has struggled with his golf the early part of this season for whatever reason, I am sure the management company weren't giving him golf lessons or caddying for him or telling him how to play golf. That's kind of his own deal," McDowell said.
"But when we are not on our game, we have a tendency to ... I am not going to say make wrong decisions, but we do question everything in our lives what we are doing and sometimes we have to make changes."
McIlroy signed a long-term deal with Horizon Sports, which stands to earn commission from the length of the recent endorsement deals.
The shake-up is another example of how McIlroy calls the shots.
His split from Chandler and ISM occurred on the flight home from Bermuda for the PGA Grand Slam of Golf, which McIlroy said was the "hardest decision I've ever had to make in my life." But he shared no details on why he needed to change.
"Sometimes to go forward in your career, you need to make a decision," McIlroy said in Shanghai after joining Horizon. "I thought it was a decision I needed to make. I felt like for four years, Chubby was the best guy. But sometimes to progress you need to have a different view and a fresh view of things. It's about me trying to play my best golf. That's all there is to it. With a new environment around me, it might enable me to play even better."
His father, Gerry, was with McIlroy during the Florida swing before heading home to Northern Ireland. McIlroy is selling his home outside Belfast and lives primarily in a house he bought in south Florida, when he's not travelling to see Caroline Wozniacki.
AP freelance writer Bernie McGuire in Bulgaria contributed to this report.