Myra Morton, 48, resented the marriage and her husband's plans to have children with the younger woman. She shot her spouse of 25 years, Jereleigh Morton, twice in the head while he slept, and blamed the crime on an intruder.
She faces about five to 20 years in prison.
Defense lawyers acknowledge the crime was intentional, but say it stemmed from the damaged psyche of a humiliated, aging woman still depressed over the death of a teenage child years earlier.
"The idea that this man was now going to marry another woman, and provide her with money that came from a dead child's lawsuit recovery, was too much to bear," lawyer Brian McMonagle said.
"That's why we were able to convince prosecutors that it wasn't a cold-blooded, premeditated murder," McMonagle said. "All the things in her life led to a breaking point."
Myra Morton spent about 20 years working at Temple University as a secretary before she and her husband, a handyman, received a reported $8 million medical settlement in 2005 over their daughter's death. They moved with their surviving daughter from a North Philadelphia row house to a $1 million home in the tree-lined suburb of Blue Bell.
The Mortons had converted to Islam about 20 years ago. Roaming the Internet
In keeping with Muslim custom, Myra Morton traveled to Morocco to bless his March 2007 marriage to 37-year-old Zahra Toural. But police say Morton grew to resent the arrangement, at least in part because her husband was sending Toural $3,000 a month.
Her husband, 47, allegedly told her to divorce him if she didn't like it, police said.
As part of the plea, prosecutors agreed not to seek more than a 10-year prison term, although the judge can go higher. Sentencing was deferred for several months while a pre-sentence report is prepared.
"Today is the first step toward really getting to the truth of the case," Assistant District Attorney Steven Latzer said Friday.
Third-degree murder in Pennsylvania involves malice, an element that separates it from manslaughter.
"We always said it was a murder case. She pleaded to murder today," he said.
The Mortons' adult daughter and her family are slated to inherit the Mortons' assets under a law that prevents killers from profiting from their crimes, Latzer said.
However, Toural recently filed a defamation lawsuit against Myra Morton, charging that she falsely told the U.S. State Department that Toural had terrorist ties in an effort to keep her out of the country.
"All she (Morton) cares about are that her daughter and her granddaughter get their inheritance, and are provided for," McMonagle said. "That is always a concern, particularly when you have so many hands reaching for this money."