Harold Rossfields Smith, former American University student and Washington-area promoter of rock concerts and fights, was convicted in federal court in Los Angeles yesterday of stealing $21.3 million from Wells Fargo bank, the largest bank embezzlement in United States history.

The jury found Smith guilty of 27 counts in a 32-count indictment that accused him of an elaborate scheme to siphon off the money and use it to become one of boxing's most flamboyant and free-spending promoters. Smith could be sentenced to up to 265 years in prison.

A codefendant, Sammie Marshall, who with Smith was an officer of Muhammad Ali Professional Sports (MAPS), was convicted on three of four counts of the indictment, which was handed down last July.

Smith, who predicted when the jury began deliberations Jan. 4 that "I will be vindicated," listened quietly as the verdicts were announced. "It's been a long battle, and it's not over," he said later. "I am a man, and the only thing I fear is what God puts in front of me."

Marshall and Smith, who is free on bonds totaling $500,000, were allowed to remain free pending sentencing March 9. Smith's lawyer, Howard Moore Jr., said there was no decision on whether the verdict would be appealed.

Assistant U.S. Attorney Dean Allison said he felt Smith was a "very, very guilty man and the evidence proved it. The jury did the right thing. Any appeal in this case would be frivolous."

Ali was not implicated in the case and had no connection with MAPS other than to lend it his name.

Smith attended American University on a partial track scholarship in the mid-1960s. He was named Ross Fields while in college and during the years immediately afterward, when he was well-known in the Washington area as a rock music promoter.

He had his name changed legally to Harold Rossfields Smith following disclosure of his true identity after his arrest last spring in Los Angeles on charges of falsifying passport information.

Harold Smith was the name he was known by when he was head of MAPS in the late 1970s. He paid huge purses to such fighters as Thomas Hearns, Gerry Cooney and Scott LeDoux to lure them away from other promoters and became a dominant figure in professional boxing.

After leaving American University, Smith operated the Sammy Davis Jr. Discotheque at 13th and E streets NW before leaving town in the early 1970s.

Both Smith and Marshall were convicted of conspiring with a former Wells Fargo Bank official, L. Ben Lewis, to divert bad checks that Smith cashed so they would not be charged to the MAPS account.

Additionally, the three conspired to manipulate the MAPS account through a system of bogus entries, making it appear the account contained more money than it did. Funds then were withdrawn from the account or laundered through the accounts of two dummy corporations controlled by Smith, prosecutors charged.

Lewis pleaded guilty to his part in the scheme and testified against Smith and Marshall.

It was almost a year ago, on Jan. 23, 1981, that bank officials first uncovered the plot. Questioned about it then, Lewis went out for lunch and did not come back, disappearing for several months.

Smith also dropped out of sight for several months, but was arrested in April on the phony passport charge and on a bad check charge from North Carolina. In revealing his true identity at the time, prosecutor Dean Allison called Smith "a bunco artist" and said he had left a trail of bad checks across the country after leaving Washington for California.

Convicted on the passport charge, Smith, 38, was sentenced to serve 40 weekends in jail.