At a news conference later in the day, Mendte said his actions grew out of a feud with Lane that began after he ended what he said was a flirtatious and improper relationship with her, including long dinners and late nights out together.
Mendte accused Lane of undermining his standing at the station. Fearing his career was on the rocks, he began monitoring her e-mails.
"I felt I was in trouble. My career, my future, my family's future was in trouble. And, this is where I got into more trouble - federal trouble," he said.
Lane's attorney, Paul Rosen, promptly disputed the account. He said there had never been an improper relationship, but that Mendte set out to wreck Lane's career because her star was rising.
"This is sick, narcissistic, sadistic - and he should pay the penalty for this conduct," Rosen said of the comments.
Mendte, 51, earned about $700,000 a year at the CBS affiliate. Rosen believes he became jealous of his 36-year-old co-anchor in early 2006 as her salary climbed to about $780,000. He bought a keystroke-logging device to get her passwords in August 2006, and intercepted e-mails from Lane's two personal and one work accounts, prosecutors said.
From January to May of this year alone, Mendte read Lane's e-mail 537 times, the FBI said.
"Five hundred times you access someone's e-mails in five months? You don't read your own personal e-mail that many times," Assistant U.S. Attorney Michael Levy said after Friday's plea.
Mendte appeared in court Friday with his wife, local Fox news anchor Dawn Stensland.
Levy said two facets of the crime make it a felony: that Mendte attempted to interfere with Lane's plea negotiations and that he leaked confidential attorney-client correspondence to the press.
Mendte, a former "Access Hollywood" co-host, faces up to five years in prison, but is likely to get much less time under federal guidelines when he sentenced Nov. 24. Prosecutors have agreed not to recommend a sentence.
Mendte accused Lane of undermining his standing at the station after the feud started, and said he felt his career was on the line. Rosen alleged that Mendte intercepted Lane's e-mails 7,000 times in a two-year period.
"I was not the source of every story about Alycia Lane or the sole source on many, but I was a source, and I was getting some of the information from her e-mails," Mendte said.
As the twice-divorced Lane's personal life became tabloid fodder, she complained to the station that her work e-mails were getting passed around, Rosen said.
"They treated her as if she was paranoid," he said. In April 2007, Mendte came upon e-mails between Lane and NFL Network sports anchor Rich Eisen.
Lane had sent Eisen a photo of her and her girlfriends at the beach - prompting Eisen's wife to send a wry response. The wife's e-mail soon appeared in the New York Post. FBI agents later found the Eisen e-mails on Mendte's home computer.
Prosecutors also suspect Mendte mailed New York prosecutors an anonymous letter suggesting Lane, based on her work history, would be loath to do community service.
"I would argue that he's the only one who could have written it," Levy said.
Lane is suing the station over her dismissal, which followed her much-publicized arrest. New York prosecutors later agreed to drop all charges if she stayed out of trouble.
She will wait to consider a possible civil suit against Mendte until after his sentencing, Rosen said.
Mendte previously worked at the NBC affiliate in Philadelphia and at stations in Chicago, San Diego and New York.