But today, the paintings are as likely to be found in basements and storage rooms, the victims of faded memories and changing tastes.

Formal oils that once lined company hallways went the way of the three-martini lunch, replaced by photographic portraits that take up less space and smack less of corporate excess. And though the names of many Seyffert subjects — Mellon, Taft, Lindbergh, Frick — are part of American history, that has not always guaranteed the images a place in homes or galleries.

A Baltimore bank that lent Robert Seyffert  a portrait of its founder for a retrospective told him he could keep it. No one seems to know what became of a portrait of Elizabeth Arden, the cosmetics company founder.

Mr. Seyffert, 57, is out to change all that. He has spent the last few years tracking down the paintings and the descendants of his grandfather’s many sitters to enlighten them about the artist, who is considered one of the best American commercial portraitists of the early 20th century.

“I want to have them feel there was a human being that did the portrait, rather than just have a picture on the wall,” said Mr. Seyffert, himself a painter. “It’s developing this idea that there was a human connection to those paintings. That connection and that sense of history is important to me.”

History has not entirely abandoned Leopold Seyffert (pronounced SIGH-fert), whose work is in the collections of the National Academy of Design, the Metropolitan Museum of Art and the National Portrait Gallery, among other museums.

In his heyday in New York, he attracted a wide circle of well-connected subjects, like the composer Fritz Kreisler, the labor leader Samuel Gompers and the corrupt Boston mayor James Michael Curley. He socialized with some, including the conductor Leopold Stokowski, and was at ease with the powerful and accomplished despite his humble background as the son of German immigrants.

The Seyfferts settled in Missouri, then Colorado. After his father died in a construction accident, Leopold moved with his mother to Pittsburgh, where he worked as an office boy for John Worthington, a geologist for Standard Oil.

In a twist worthy of Dickens, Mr. Worthington noted the young man’s artistic talent and became his benefactor, helping him attend the Pennsylvania Academy of the Fine Arts, where he studied with celebrated artists including William Merritt Chase. Leopold later traveled to Spain and studied the paintings of Goya and Velázquez, returning about 1915 to embark on a career as a portraitist.

Robert Cozzolino, the curator of modern art at the Pennsylvania Academy, sees Leopold Seyffert as a bridge between the Gilded Age portraitists of the late 19th century and the artists of the 1920s and ’30s who nudged the boundaries between the real and the imagined. The painter, he said, distinguished himself with bold brushwork, or portraits in which the body blends into the background.

“Seyffert put more of himself and more art into those portraits,” Mr. Cozzolino said. “It was the subtle things that set him apart, the way the sitters posed, the things he included in the portrait. He treated the paint not as this descriptive thing, but there are all these little flourishes he gets away with. It just set him apart from the deadpan realist style that persisted in the ’20s.”

Changes in art and corporate culture eventually pushed Seyffert to the side, well before his death in 1956.

“Modern art sort of killed my grandfather,” Robert Seyffert said. “He was seen as too traditionally academic.”

Yet it never killed the grandson’s curiosity about the artist, who in addition to his more than 400 oils, drew scores of charcoal portraits, many while summering in Maine. One was of Julius Rosenwald, who helped build Sears Roebuck into a merchandising giant, then became a generous benefactor to African-American colleges and other causes.

With little more than some Internet sleuthing, Mr. Seyffert located Rosenwald’s granddaughter, Sandra Stern McIver, and e-mailed her a photograph of the sketch.

“I never knew it existed,” Ms. McIver said. “I have a lot of photographs of him, but none captured his humanity the way this does.”

She bought the drawing and hung it in her home office in Mendocino, Calif. “When I look at it from my desk, he’s looking in my eyes,” she said. “Sometimes he stops me from being childish. Sometimes he picks me up when I’m discouraged. Sometimes he teaches me not to be selfish. And other times I just feel love, which is pretty interesting, since it’s from somebody I never met.”

More recently, Mr. Seyffert connected with Harper Sibley 3rd, a descendant of the founder of Western Union. Leopold’s portrait of Mr. Sibley’s grandfather, who shared his name, is still treasured by the family.

“It speaks to a bygone time that Leopold Seyffert was able to capture in this portrait,” said Mr. Sibley, who lives in Westchester County and Florida. “This was a class of men who really tried to spend their time in community activities, as his wife did. That portrait reminds us of the obligation we all have to do that, regardless of our station.”

Working with only a catalog from a 1914 show in Rochester, Mr. Seyffert hunted down a portrait of Dr. Franklin Mall, a renowned professor of anatomy at Johns Hopkins University. The painting, which was in storage at the university, had vexed archivists.

“We were very interested to have the identity of the artist,” said Nancy McCall, director of the archives for the Hopkins Medical Institutions. “It is an extraordinarily well-done portrait. The head and hands are exquisite.”

Some portraits have landed in even more surprising places. Mr. Seyffert recalls visiting a West Hollywood antiques shop in the late 1990s and mentioning his grandfather’s work. The owner said he had a painting in that style, and showed him.

“There was my grandfather’s painting of Elsie Whelen,” he said. “She had once been married to a wealthy real estate mogul who owned several blocks on Fifth Avenue. I couldn’t afford to buy it. He sold it to Ralph Lauren, and it’s now hanging at his Milan store.”

Still, Mr. Seyffert has about 30 paintings by his grandfather. A stunning full-length portrait of Frances Maris, whose husband was president of Du Pont Motors, greets visitors to his Hunts Point studio, where he paints Hopperesque streetscapes.

High on one wall is a large canvas of country life that Leopold painted in Spain. Below, a Spanish dandy in a red jacket stands ready to go to the fair. Mr. Seyffert says he can feel the life that pulsed through his grandfather’s brushes.

“I wish I could speak with my grandparents now,” said Mr. Seyffert, who was only 4 when his grandfather died. “I have so much more to say to them now than in my 20s. Before, I was in a hurry. But now I could sit with them in a room and talk with them for hours.”