Church law requires that Mahony attend, the Archdiocese of Los Angeles said.
Father Albert Cutie, an Espiscopal priest who studies the Vatican, said it would be impossible to exclude every cardinal with a hand in the church's vast sex-abuse problem.
"Unfortunately, if you were going to tell me no one can go to the conclave who has part in any type of cover up, you would probably exclude every cardinal in the church, because unfortunately that's the way the church is operated," he said.
As if the controversies over O'Brien and Mahony were not enough, two Italian publications reported over the weekend that Benedict had decided to resign not because of age, but because of a brewing scandal over the blackmail of gay priests by male prostitutes in Rome.
Benedict received a 300-page report in December detailing the possible blackmail, la Repubblica newspaper and the Panorama news weekly reported, citing an unidentified senior Vatican official and dozens of unnamed sources.
The Vatican emphatically denied the allegations this weekend, with Secretary of State Tarcisio Bertone criticizing a rash of "often unverified, unverifiable or completely false news stories" as the cardinals prepare for their conclave.
Cardinal Velasio de Paulis, one of the men who will help elect Benedict's successor, called the claims "guesswork and imagination."
"There is no proof and these allegations only serve to create a climate of division that helps no one," he said.
While no one outside the Vatican has seen the document that purportedly details the claims and Vatican officials have not confirmed it exists, Allen said such a claim is not improbable.
"To me that passes the smell test," he said.
Amid the scandal, the Vatican still has a transfer of power to manage.
On Monday, Lombardi said it remains unclear when the gathering of church leaders who will elect the next pope will begin.
While Benedict issued an order Monday to allow the election to begin sooner than the 15 days after the seat becomes vacant mandated by church rules, the date for the election will be set by the cardinals when they first gather, Monsignor Pier Luigi Celata said Monday at a Vatican press briefing.
It still must happen within 20 days of his resignation, the pope said.
After his retirement, Benedict is expected to head to the pope's summer residence in Rome before eventually settling in a monastery in Vatican City. Church officials have said he will seek no influence over the election of his successor, or over management of the church.
Among other issues, Vatican officials are still trying to work out what Benedict will be called in retirement. One suggestion is "pontifex maximus," Celata said. The term can be translated as "supreme bishop."
Vatican officials hope to have an answer next week, Lombardi said.