Former UN weapons inspector Scott Ritter has been found guilty of unlawful contact with a minor following an online sex sting operation.
Ritter, an outspoken critic of the war in Iraq, had a sexually graphic online chat in 2009 with an undercover police officer posing as a 15-year-old girl named Emily.
The 49-year-old was convicted on six of seven counts.
Ritter's lawyer said his client did not think he was speaking with a minor.
Ritter, a former US Marine, initiated a conversation through a Yahoo chatroom in February 2009 with police detective Ryan Venneman who was posing as the teenage girl.
Mr Ritter then performed a sex act on himself in front of a web camera, despite the officer's repeated claims that he was 15-years-old.
'Troubled and offended'
Ritter remained silent as the verdict was delivered, while his twin 18-year-old daughters, who were in attendance in the court in Pennsylvania, sobbed.
"The jury reached the right decision," Assistant District Attorney Michael Rakaczewski said after the verdict.
"They saw the case for what it is and the defendant for what he is and what he did," he added.
On Tuesday, prosecutors played a graphic 20-minute video of the exchange between Mr Ritter and Mr Venneman for the jury.
Defence attorney Gary Kohlman acknowledged in his closing argument on Thursday the jurors were likely "troubled and offended" by the graphic chat and video of the conversation.
A decade ago, Mr Ritter was charged in New York with trying to set up a meeting with an undercover police officer posing as a 16-year-old girl.
Those charges - which Mr Ritter claimed were politically motivated - were dismissed after he completed six months of probation.