He is such a deft stand-up comic that he is no less funny when he sits down. One starts chuckling as soon as he enters to the regal strains of the ''Masterpiece Theater'' theme and suggests that the stage is bare because you have come to see him, not furniture. He segues through an astonishing quantity of themes. His style is not that of Henny Youngman, which both will be grateful for. No one-liners. No stories, either, a la Sam Levenson. No? What then? No jokes, no stories? From this he makes a living as a comic?
Mr. Mason is an essayist, a commentator, an observer of how people and society work. Maybe he is a verbal cartoonist. Maybe he's a glass of tea. No matter, he is very funny and anyone attempting to read meaning into what he is saying should be condemned to a siege of heartburn without benefit of seltzer.
His timing is flawless, and his material is clever. He delivers it in the resonant New York diction, roller-coastering over inflections that rise and fall with a tidal surge equal to that of the Bay of Fundy. He is delivering, on Broadway, the same sort of performance that has been his stock in trade all these years in nightclubs, in the mountains, on the prairies, on television. He even singles out people in the front row for mild insult, just as a comic does in a club, and he establishes an intimacy.
The show works because, unlike many other things that make it to Broadway, he has not succumbed to the temptation to be pretentious.
His range is limitless, within the bounds of a Jewish comic who brooks no pretention and even does a bit about the Jewish types who are afraid that he is too Jewish. He makes obeisance to nightclub ritual with the tired and familiar bits about timid Jewish and macho Italian husbands and aggressive Jewish and self-effacing Italian wives, the somewhat offensive but clubby ethnic shticks that are rooted in New York tribalism. Just as you are about to give up on him, Mr. Mason come up with a good double-take line designed to ward off the whammy of stereotyping and prejudice. Clever fellow.
He is positively hilarious when he flays television weather forecasters who are strong in the big outlook but when it comes to specifics give you an 80 percent chance of rain (''Did anyone ever buy 80 percent of an umbrella?'') or the airport temperature (''Who lives at an airport?'').
And then there is Iran and the President, ''God bless him.'' Mr. Mason does a job here. The President is unlike any other President we've had. Other Presidents looked worried when there were crises. Mr. Reagan jumps on a horse, eats jellybeans and says ha-ha. After everybody in the White House says they know nothing about the arms sales, the President promises co-operation.
''Just as we say we're cooperating, they start asking questions,'' Mr. Mason has the exasperated President saying.
The show, two hours overall, including intermission, is divided into several segments, each with a different backdrop that sparks a different theme. Mr. Mason's litany takes on sex, money, psychiatry, Beverly Hills, money, Israel, hookers, money, Einstein, gentiles, money, Jews, Broadway and money: ''I have enough money to last me for the rest of my life - if I don't buy anything.''
He also does very funny mimicry of James Cagney, Henry A. Kissinger, Senator Edward M. Kennedy, folk singers, pop singers. Mr. Mason does not do windows, but he does everything else, and you can save yourself the drive to Monticello. The mountains have come to Manhattan.