"I do this in order to devote all of my time and energy to restoring the excellent reputation my wife, June, enjoyed until recently as one of the most outstanding educators in the state and, indeed, the nation," Brown said in the letter.
Nutter's press secretary, Mark McDonald, praised Brown for serving the city "with energy, intelligence, and distinction."
"He was a valued board member who will be missed," added Ethics Board Chairman Richard Glazer.
A federal grand jury indicted Dorothy June Brown, 75, the founder of four area charter schools, on charges that she defrauded three of the schools of $6.5 million. Four school administrators were also charged.
The 62-count indictment accused her of wire fraud, conspiracy to obstruct justice, obstruction of justice, and witness tampering.
The government alleges she created and used two private educational management companies to defraud the schools.
The U.S. Attorney's Office also accused Brown and her codefendants of fabricating documents, including contracts and board resolutions, meant to justify the payments to the management firms, in response to questions from federal investigators.
The day the indictment was announced, Dorothy June Brown's lawyers issued a statement saying the former Philadelphia School District principal denied all the charges "and intends to vigorously defend herself against these unfounded charges."
Contact Bob Warner at 215-854-5885 or firstname.lastname@example.org.
Inquirer staff writer Martha Woodall contributed to this article.