To judge by reports at the time, he had been on a spree that should have made him as rich as Bill Gates. It was said he had stolen details of 120,000 credit cards from an internet company, plus $80m (pounds 47m) in software from companies including Motorola, Sun Microsystems, Nokia and Fujitsu.
His downfall began - so the story goes - at Christmas 1994, when he used telephone lines to break into the home computer of Tsutomu Shimomura, a supercomputer expert based in San Diego, California. Shimomura pitted his skills against Mitnick to track him to his lair.
Mitnick was taken to jail to await trial for his alleged deeds; pictures from the time show a chubby, sallow man looking shyly at the press cameras.
Four years later, Mitnick is still in prison. But he has not been tried for those crimes. He still awaits a trial that has been delayed, and delayed, and delayed again. He is believed to be the longest-serving remand prisoner in the US.
While on remand he has been sentenced once for other crimes (including a 1992 violation of probation), but he was adjudged to have already served the 22-month sentence. The trial for those 1994 events is not even due to start until April.
"The US legal system is very different, but I don't believe he's such a menace that he should be held without trial," said Steve Gold, news editor of Secure Computing magazine. "It's inexcusable. I'm a member of Liberty and Amnesty. Whatever the crime might have been, it's inappropriate."
Mr Gold has a special perspective. He and Robert Schiffreen were the first people in Britain to bring the word "hacker" to public attention in the 1980s, when they accessed the Duke of Edinburgh's e-mail letterbox on British Telecom's Prestel system.
"When we did that, it wasn't illegal," said Mr Gold. "Mitnick's problem was that he didn't change with the times."
Yet Mitnick, now 35, has not had much chance to change, inside the Los Angeles Metropolitan Detention Center.
From time to time he has been consigned to solitary confinement; he spent New Year's Eve (and the following two nights) sleeping on the floor because he was being shifted between areas of the prison for "security reasons" but there was no bed prepared in his new cell.
He is barred from a computer or a telephone - and certainly never allowed to put both together.
His supporters in cyberspace have begun a "Free Kevin" campaign, setting up a website (www.kevinmitnick.com) and by hacking into other sites to publicise the message. Mitnick, through his lawyer, has gently suggested they stop hacking.
Last June he waived his right to a speedy trial, to have his trial date delayed again, because his attorney said that the US government had been slow in "discovery", making prosecution evidence available.
Mitnick and his lawyer, Don Randolph, have a mountain to climb. In computer format, the case against Mitnick amounts to 1,400 pages of witness statements and nearly 10 gigabytes - 10,000 megabytes - of electronic evidence.
"Nobody understands," he told an American reporter last December in one of his first interviews since the arrest. "I'm basically being forced to do it. I have two choices: go to trial with an attorney who isn't prepared enough to competently represent me, or waive my right to a speedy trial."
Mr Gold feels Mitnick is being made an example, to show other hackers what they may face if they follow in his footsteps. "Arguably, he is or was one of the best hackers in the world," he said. "But he's been caught red-handed three times, and a leopard doesn't change its spots. The fact he keeps doing it suggests he's beyond redemption."
Mitnick is said to have had a bleak childhood. He grew up in an impoverished area of the San Fernando Valley in California; his parents divorced when he was three, leaving him with his mother, who had to work long hours in a restaurant.
He was a shy, overweight adolescent, but he got his first taste of beating the system at 13 when he worked out how to punch his bus tickets for free trips. Then he moved on to ham radio, and phone "phreakers", who exploit weaknesses in the phone system to make free long-distance calls.
His crowning success - and the one that really brought him to the authorities' attention - came in 1979, when he broke into the electronics of the North American Air Defense Command (Norad). That was the inspiration for the 1983 film War Games, though no damage was discovered.
Then he tried bigger and bigger pranks - and was caught, jailed, and given probation. He broke the conditions and was jailed again. This time round, bail is repeatedly refused.
Mr Randolph said: "To the extent that Kevin is a "hacker" he must be considered a purist. The simple truth is that Kevin never sought monetary gain from his hacking, though it could have proved extremely profitable.
"Nor did he hack with the malicious intent to damage or destroy other people's property. Rather, Kevin pursued his hacking as a means of satisfying his intellectual curiosity and applying Yankee ingenuity."
Mitnick, facing a potential total of 200 years in jail on 25 charges of computer damage and wire fraud, spends the few hours a week he is allowed in the prison library engrossed in legal textbooks.
With a trial date approaching, at least he can hope that the end is in sight - though he has probably thought that every day for the past four years.
Mr Gold thinks enough is enough. "For all that he's done, there are despots and mass murderers out there who have suffered a lot less than Kevin."Reuse content