"We are disappointed, think the jury is wrong, and we will be appealing," said Patricia Glaser, Christensen's attorney and law partner.
Pellicano and Christensen were accused of recording Lisa Bonder Kerkorian's phone conversations in an effort to disprove her claims that MGM mogul Kirk Kerkorian was the father of her young daughter.
"This case uncovered corruption by the wealthy and influential, and today's guilty verdicts render assurance that the justice system cannot be bought by those with money and power," said Salvador Hernandez, assistant director in charge of the FBI in Los Angeles.
Each of the 64-year-old defendants faces up to 10 years in federal prison and $500,000 in fines when sentenced by U.S. District Judge Dale S. Fischer.
Pellicano was convicted of illegal wiretapping and racketeering in a separate case earlier this year. He was remanded to jail after the latest verdicts were announced and is to be sentenced in both cases Sept. 24.
Christensen, who is free on $100,000 bond, is to be sentenced Nov. 17. He was ordered to surrender his passport and instructed not to travel outside of central California without permission.
The jury began deliberating Wednesday but had to start again Thursday when Fischer dismissed one juror for making questionable comments about the severity of the charges in the jury room and then lying about making them.
The case was built on recorded conversations between Pellicano and Christensen in which prosecutors said they discussed Bonder Kerkorian's private phone calls. Jurors heard a playback of several conversations during deliberations.
Glaser stressed to jurors that no actual recordings of the phone calls were ever recovered.
Bonder Kerkorian, a former tennis pro, was married to Kerkorian, now 91, for 28 days in 1999. Christensen paid Pellicano $25,000 up front and promised $100,000 more if he could identify the true father of the girl, prosecutors said. DNA tests later showed movie producer Steve Bing was the biological father.
Kirk Kerkorian testified briefly during the trial, saying he had no knowledge of any wiretapping being used in the paternity case.