He was 44.
Lighty was found dead outside his Bronx, N.Y., apartment with a gunshot wound to the head, the New York Daily News reported.
Police sources told the newspaper Lighty killed himself after an argument with his estranged wife, Veronica.
Insiders also told the Daily News Lighty may have been dealing with financial issues, including a $5 million federal tax bill.
Numerous celebrities took to Twitter to express their shock and grief over his death.
"Rest peacefully Chris Lighty, my prayers go out to family and loved ones! Dear God please have mercy," singer Rihanna tweeted.
Musician and TV-radio personality Nick Cannon, who is married to Carey, said he is "devastated right now."
"I can't believe my big brother Chris Lighty is gone," Cannon tweeted. "He was a pioneer, a mentor, and a great friend RIP. I truly believe to be absent from the body is to be present with the Lord, but I just feel like Chris's work wasn't finish on earth."