Mobley said doctors told him he faced significant risks if he kept playing. The 11-year veteran said by walking away now, he could live a long life.
Mobley, 33, announced his decision at a news conference at the Knicks' training center, where he confirmed he has hypertrophic cardiomyopathy. The condition causes the heart muscle to thicken, making it harder to pump blood, and he said he had no choice but to end his career.
"The specialists I've seen made it clear that my heart condition has gotten worse and I couldn't continue to play professional basketball without putting my health and life in serious danger," Mobley said. "As much as I want to keep playing in the NBA, I have no choice but to follow the advice of my doctors and step away from the league."
Hypertrophic cardiomyopathy is the leading cause of sudden cardiac death in people under 30 years old and was linked to the deaths of former Boston Celtics forward Reggie Lewis and Loyola Marymount star Hank Gathers.
The Knicks acquired Mobley last month from the Los Angeles Clippers -- a trade that may ultimately have saved his life. An EKG during his physical showed an irregularity with the heart, which Mobley already knew existed. The Knicks decided to perform an MRI exam, which revealed the more serious condition that previously had gone undetected.
Mobley then saw four specialists around the country, who performed additional tests and provided him literature about the disease that convinced him to stop playing.
"The doctors said to not chance it and I feel as though they're right, having an 8-year old son, having a long life ahead of me, it's the smart thing," Mobley said. "It's a tough thing to swallow, but things in life happen, but you have to keep going."
Mobley averaged 16.0 points in 11 seasons with the Houston Rockets, Orlando Magic, Sacramento Kings and Clippers. He was expected to become the Knicks' starting shooting guard after the Nov. 21 trade.
The Knicks could have voided the deal because of the health concerns but waived the physical requirement because the trade allowed them to move Zach Randolph's contract, freeing salary cap space for the summer of 2010.
"I thought this would be the perfect trade with him in it, but I would never put a player out there that's at the risk that he would be at," Knicks president Donnie Walsh said.
"None of this is as important as somebody's life. None of it. Period," Walsh said. "So I'm glad that we had a doctor that put him through tests that showed it, because the risk was there."
Walsh said he wasn't sure yet what options the Knicks had to fill the vacant roster spot and would begin researching it Friday.
Mobley said he'd thought about the scene of Gathers collapsing on the court during a game and said it factored into his decision.
"Say if you were to play, the worry, the worrisome of people watching you, if you were to fall, or it's just an elbow or just an ankle, that's scary. And then every single day just being scared for you, I think that's a selfish thing, also," he said. "Even though you love something so much, and I am in love with basketball, but sometimes you have to get a divorce."