A representative for Phillips also announced the former Mets general manager was entering a treatment facility "to address his personal issues."
Phillips' acknowledgment Wednesday of his relationship with 22-year-old Brooke Hundley was splashed across the New York tabloids for days, embarrassing the Bristol, Conn.-based sports giant.
"Steve Phillips is no longer working for ESPN," network spokesman Josh Krulewitz said in a statement. "His ability to be an effective representative for ESPN has been significantly and irreparably damaged, and it became evident it was time to part ways."
Phillips had taken a leave of absence after the affair became public. Krulewitz declined comment when asked Sunday night about Hundley's status with the company.
Steve Lefkowitz, Phillips' representative, said in an e-mail that his client "is voluntarily admitting himself to an inpatient treatment facility to address his personal issues, and informed ESPN of his plans Friday."
According to a police report filed in Wilton, Conn., Hundley began calling Phillips' wife, Marni, on Aug. 5 after he broke off the affair and sent her a letter graphically describing their relationship and the 46-year-old Phillips' birthmarks.
Marni Phillips called police Aug. 19 when she came home to find Hundley in her driveway. "I knew instinctively that this was the woman Steve was involved with and I was terrified," she wrote in a statement to police.
Hundley also contacted Phillips' 16-year-old son through his Facebook account, according to the police report.
"This woman has clearly displayed erratic behavior and delusional tendencies," Steve Phillips said in a statement to police.
Phillips signed a statement to police that he would not press charges. The status of the police investigation was unclear Sunday. The report indicated a detective planned to interview Hundley this week when she returned from vacation.
Marni Phillips filed for divorce Sept. 14, according to court records.
In 1998, Steve Phillips admitted having sex with a Mets employee, who sued for sexual harassment. That case was settled out of court. Phillips was fired by the Mets in 2003.
Messages seeking comment were left Sunday for Phillips and Hundley.
ESPN has been troubled by a series of workplace issues involving alleged misconduct by its television personalities.
In 2006, baseball analyst Harold Reynolds was fired after a female intern complained about what he called a "brief and innocuous hug." Reynolds sued and settled with the network last year.
Last year, a judge dismissed a lawsuit filed by a makeup artist who accused hosts Jay Crawford and sports writer Woody Paige of groping and propositioning her on the set of the now-defunct show "Cold Pizza."