Ms. Rogoff’s dream suggests the enigma of Mr. Williams’s largely undocumented life, much of it spent in New Jersey. She has been working for the museum for 21 years and researching the artist for 18 of them, drawn to his story because “he’s just so good,” she said.

“Even now, after all these years, if I turn around and look at one of his portraits, it still catches me,” she said.

“But he’s a mystery man,” said Ms. Rogoff, 50, of Ocean Township.

A lot of the mystery surrounding Mr. Williams has centered on finding and cataloging his work. Of 272 known portraits, few are signed; the sitters themselves may have written his name on some pieces. But he is not so much of a mystery that his paintings — richly detailed portraits, mostly pastels, from 1815 to 1835, of middle-class New Jersey and New York residents — have escaped the notice of galleries, museums and collectors.

“A lot of collectors who specialize in folk art like to have at least one Micah Williams,” Ms. Rogoff said during a recent tour of the exhibition, which includes 67 portraits in three galleries, as well as one of the artist by an unknown painter. The historical association owns 22 Williams pastels and one oil painting. The other works were lent by private owners in New Jersey, New York and Pennsylvania and museums including the Zimmerli Art Museum at Rutgers University in New Brunswick.