Ambrose is accused of tarnishing his marshal's badge and betraying the trust that he swore to uphold.
A jury Monday afternoon heard the government explain how Mr. Ambrose was giving out top secret information that ended up in the hands of Chicago mob bosses, an allegation that Ambrose himself doesn't totally deny.
In district court on Monday, he looked the part of a clean-cut lawman, the same image John Ambrose projected in TV appearances as he and the U.S. Marshal service beat down doors and arrested some of the nation's most feared fugitives.
That picture of law and order was not the one painted by federal prosecutor Marcus Funk in Monday's trial opening.
Funk told the jury that Ambrose leaked confidential information about hitman-turned-witness Nick Calabrese. That information found its way to a Michigan prison where mobsters James and Michael Marcello discussed it in a 2003 meeting.
Ambrose, known to mob bosses as "The Babysitter" because he was guarding Nick Calabrese, is accused of funneling information to Outfit leaders.
In conversations recorded by the FBI, mob bosses identified their snitch in the marshal service only as the son of former Chicago policeman who had been convicted in the Marquette Ten corruption case who died in prison.
Authorities say that led them to John Ambrose, whose father Thomas was the Marqutte Ten cop.
When the FBI realized they had a leak and that the evidence pointed to John Ambrose. They questioned the decorated deputy marshal.
At first they say he denied it all, then admitted most everything.
Ambrose' lawyer Frank LaPuma told the jury that his client messed up and screwed up but that it's not how it looks and that he had no criminal intent. LaPuma suggested that the federal investigation of Ambrose was a comedy of errors.
Convicted mobster Mickey Marcello will be the major witness in the case. Marcello, a party to the jailhouse conference with his mobbed-up brother, has been given immunity to testify.
Neither Mr. Ambrose nor his attorney would comment on the way out of court on Monday afternoon.
Ambrose, who celebrated his 42nd birthday on Sunday, is on suspension from the marshal service.
No one was hurt as a result of his alleged leak.
The case could last up to three weeks.
Several marshal service deputies who are responsible for protecting important witnesses, will be allowed to testify from behind a screen-to keep their identities secure.