The couple signed the brief message "Ed and Midge." They did not use the word divorce.
"We wanted to let you know that we have decided to embark upon this next phase of our lives by living separately," they wrote.
"This has been a difficult decision, but we both believe it is the right thing to do. Our parting is amicable, and we will remain friends and continue to be active in our community, sometimes together, sometimes separately. We hope you will respect our privacy, and our decision, and wish us well," the e-mail said.
The note, first reported by the Philadelphia Daily News, concluded: "p.s. Please do not hesitate to include both of us in social occasions as we will not find it awkward or uncomfortable."
Rendell, 67, a former Philadelphia mayor, stepped down as governor on Jan. 18 after serving the maximum two terms allowed by law.
Since then, he has made it clear he intends to remain busy, announcing plans to rejoin the Philadelphia law firm of Ballard Spahr, work as political analyst for NBC and MSNBC, and write a weekly sports column for the Philadelphia Daily News.
Marjorie Rendell joined the federal bench in 1994 as a U.S. district judge in Philadelphia. She was later elevated to the 3rd U.S. Circuit Court of Appeals in Philadelphia.
The Rendells have a grown son, Jesse, who graduated from Temple University Law School in 2009 and works for a Philadelphia law firm.
In June, Rendell angrily denied rumors of an extramarital affair after Philadelphia magazine published an article about gossip involving him and Kirstin Snow, a former beauty queen who worked for Rendell as director of Commonwealth Media Services, a state agency that provides multimedia services for training, tourism and other purposes.