"I WAS the most overrated man in the world," confesses banjo player, vaudevillian and Ziegfeld Follies star Brooke Johns. "I made more money for knowing less about what I was doing than any other man you'll meet. You might say I took a course in ignorance and graduated -- valedictorian." Johns, 90, may have given up show business 50 years ago, but he still knows the value of a punchy one-liner.

Tonight, when Johns performs in "The Pageant of American Song" at the National Theatre, it will mark something of a homecoming for him. Sixty years ago he appeared in the Ziegfeld production "The Comic Supplement" at the National with W.C. Fields. "It was a helluva show, 45 showgirls," Johns says. One night Fields' valet, a midget named Shorty, neglected to button Fields' fly. When the comedian walked on stage his red flannel undies sent the audience into gales of giggles. He looked like a "stop light" from the waist down, recalls Johns, who gallantly escorted Fields to the wings. Later, "you could hear Fields cursing at Shorty down on Capitol Hill."

Today, Johns and his wife of 58 years, Hazel, live quietly on the grounds of their own country club outside of Rockville. It was part of a "mile of property I bought in 1925 for $36,000," says Johns. How a poor country boy from Georgetown raised that kind of money is a story of dreams and good fortune.

As Johns tells it, no one knew what to make of him as a teen-ager, only that he was bound to be a success at something. The president of the Georgetown Preparatory School told his father precisely that when Johns received his "unconditional release" after two years of failing grades. Shortly afterward, Johns convinced his father that his rudimentary skills as a banjo player and his outgoing personality were enough to make a name for himself on stage. He left for New York -- despite the wishes of his aunts and grandmother, who thought he should enter the ministry. In no time at all, Johns was a flop.

Undaunted, he returned to Washington, where things began looking up. One engagement led to another and soon Johns was strumming away at his banjo in Palm Beach, Fla., where Ziegfeld and other show biz luminaries were vacationing.

Accepting an invitation from a Broadway producer, Johns landed a role in the Broadway smash "Tangerine," and nightclub work followed. At one point in New York in the 1920s he was holding down four jobs simultaneously and taking home $6,300 a week. The Ziegfeld Follies and a lengthy stint on the B.F. Keith vaudeville circuit were among his more lucrative assignments. "Ziegfeld was a sharp cookie," Johns says. "I asked him why he never talked to me in Florida and he said, 'Nobody knew you then.' "

Before Johns abruptly retired from show business on his 40th birthday -- as he had promised his aunts and grandmother -- he worked with vaudeville's elite. His conversations are now peppered with references to Al Jolson, Eddie Cantor, George Burns and Buddy Ebsen, as well as amusing backstage anecdotes.

And he remains stage-struck. "I used to say to Cantor, 'Eddie, you're so damned stage-struck, you'd kneel down in front of a cab with one light out if it was coming at you to sing a "Mammy" number.' Once you've been on stage, you never get over it. Not if you're 600 or 700 years old."

Johns continues to perform, mostly at hospitals and schools these days, and he's recorded two albums in recent years. "My right hand is nearly paralyzed from playing the banjo -- I played a million miles on it," he says. "But my voice, thank God, is in pretty good shape." His current album, "Return of the Roaring Twenties, Vol. 2," is an engaging, high-spirited collection of tunes from the period.

In his prime, Johns collected enough reviews and memorabilia to cram the 25 scrapbooks he's assembled. But some of the words of encouragement he cherishes most were never put down on paper. He remembers, for instance, talking to Will Rogers shortly after a Ziegfeld performance. Rogers was impressed by the electricity and charm Johns projected on stage and asked him to dinner. "Is it true," Rogers asked, "that you send half of your paycheck to your mother and father in Georgetown every Saturday night?"

"I said, 'Yes, sir.' He said, 'Is it true that you don't smoke and don't drink?' I said, 'Yes, sir.' 'All right,' he said, 'if you continue to go through life like you're doing now, you're going to be a tremendous success.'

" 'Oh,' I said, 'Mr. Rogers, I can't play the banjo much. I can't sing much. I can't dance much.' He sat back in his chair and didn't say anything for a minute and a half or two minutes.

" 'Brooke,' he said, 'will you do me a favor?'

"I said, 'Yes sir.'

"He said, 'Never learn!' "