NINETY-THREE WORKS of art--among them six Monets, two Gauguins, two Cassatts, a Renoir, a van Gogh and one of the last privately owned paintings by Seurat--have been given by Paul Mellon to the National Gallery of Art, the museum founded by his father.

President Reagan announced the gift last night following the Andrew W. Mellon dinner in the gallery's West Building.

The latest Mellon gift--of 18th-, 19th- and 20th-century European and American art--includes 50 paintings, 24 sculptures and 19 prints and drawings. Almost all the paintings are by now familiar to the museum's public, for Paul Mellon placed them there on loan many years ago.

Gallery officials yesterday declined to estimate the cash value of the gift. But it is almost certain that its most important paintings--Georges Seurat's "The Lighthouse at Honfleur" (1886); Mary Cassatt's "Little Girl in a Blue Armchair" (1878) and "Child in a Straw Hat" (1886); the early Vincent van Gogh, "Flower Beds in Holland" (circa 1883); Paul Gauguin's "Landscape at Le Poldu" (1890); and the best of the Monets--would each fetch seven-figure prices if auctioned today.

Cassatt's "Girl in a Blue Armchair" is special in part because Degas helped paint its background. The Seurat is important, too, because he completed fewer than 50 pictures. The six Monets, which survey his career from 1867 to 1904, include a pair of pictures of Waterloo Bridge, London, one at sunset, one at dusk, both of which were painted from a room he rented in the Savoy Hotel on the Thames embankment.

This is the largest single gift to come to the gallery since the Chester Dale bequest of 1962. It "magnificently strengthens the gallery's collection in a variety of fields," said J. Carter Brown, the gallery's director.

The 10 old British paintings in the Mellon gift--by John Crome, William Hogarth, Henry Fuseli, Richard Wilson, Joseph Wright of Derby and other English artists--fill important gaps in the gallery's collection. The Mellon gift includes, as well, five oils and 19 prints and drawings by the American George Bellows, two pictures by Paul Klee, a large Joan Miro', two Kenzo Okadas, a 1952 Nicolas de Sta el and three oils by Jacques Villon. Among the 24 sculptures given are seven Maillols, a Renoir terra cotta, an Alexander Calder stabile, a granite Henry Moore and seven bronze reliefs by Giacomo Manzu .

Mellon yesterday explained the timing of his gift by saying, "After all, I'm getting older." Though the gift is his, the labels on the objects will say they came from the collection of Mr. and Mrs. Paul Mellon. "We call it that," he said, "because Bunny has had an enormous impact on the collection. If we consider just the French pictures, I'd say perhaps 20 percent of them were paintings she selected. Twenty percent were pictures I was particularly fond of. Picking the other 60 percent was really a joint endeavor."

Mellon, 75, is the gallery's board chairman and its major benefactor. Not only has he given pictures in the past--among them three Ce'zannes, a pair of Canalettos, 351 American Indian paintings by George Catlin and first-rate paintings by Degas, Manet and Gauguin--he also has provided huge amounts of cash.

In the 1970s, Mellon, his late sister, Ailsa Mellon Bruce, and the Andrew W. Mellon Foundation provided nearly $100 million to build the gallery's East Building, designed by I.M. Pei. Since then, he has given an additional $4.5 million to help remodel the West Building's ground-floor galleries, which open Thursday to the public. He also has contributed $5 million to the gallery's Patrons' Permanent Fund.

And the National Gallery is not the only art museum Mellon now supports. He gave an English art collection--1,800 paintings, 7,000 drawings, 5,000 prints, 16,000 rare illustrated books, 10,000 reference volumes, 60,000 photographs--and a $12.5-million museum to house them, designed by Louis Kahn, to Yale University in 1977. With collectors Sydney and Frances Lewis, he also is helping build a new West Wing at the Virginia Museum of Fine Arts in Richmond. In addition to the $6 million he has provided for construction, Mellon plans to give to that new museum the 59 French pictures--by Degas, Gauguin, Ce'zanne, Picasso and other renowned painters--that now are there on loan.

Asked how he decided on the distribution of his gifts, he said, "I look at the museums, consider what they need, and then try to be fair."

Despite the gifts he's made, his personal collection is still large. A number of his works of art--among them a Manet, three small Peto still lifes, a suite of abstractions by the late Mark Rothko and 17 wax sculptures modeled by Degas--are now on loan to the National Gallery of Art.