From June 15 to October 7, 57 years after his death, the first major exhibition of John Prentiss Benson’s paintings will open at the Portsmouth Athenaeum, a private library devoted to seacoast history and artifacts at 6-8 Market Street. The show is entitled “A Retrospective Exhibition: The Artistic Legacy of John Prentiss Benson.”

Inspiration for the athenaeum exhibit goes to Nicholas Baker, whose wife, Joan, was a Benson granddaughter. Since Joan’s death in 2000, Baker has devoted his energies to compiling and editing the works of John Prentiss Benson.

At the age of 56, architect John Prentiss Benson, younger brother of Frank Benson, took up the brushes he had used only idly – and became a full-time professional painter. “He took up painting, and he took it up with pretty much immediately successful results,” said John G. Hagan, a Boston art dealer, expert in early Twentieth Century American art and guest curator for the athenaeum show.

Benson did most of his painting in Kittery at a handsome old home called “Willowbank,” which he bought in 1925. Benson’s principal subject was the sea – right off the front lawn of his home in Kittery.

As a painter, Benson quickly became known as the “unrecognized younger brother” of the more famous Frank Benson, a leading painter from that early Twentieth Century school of American impressionists. But Frank had a running start.

Margaret M. Betts tells the story in The Artistic Legacy of John Prentiss Benson, a book compiled and edited by Nicholas J. Baker (see www.johnpbenson.org). She said both Benson boys, growing up the sons of a merchant in Salem, Mass., in the 1880s and 1890s were interested in studying art. But, when it came time to choose a career, Frank chose art and John, three years younger, was advised one artist in the family was enough. He dutifully went into architecture. Although he built a very successful business over the years, it was Frank who became famous in the rarified world of fine art.

John Benson’s self portrait shows the artist, calm and in command, wearing a stiff white collar, proper red tie and dark suit. An artist’s smock and a barely visible brush hints at his true calling.

Museum hours are Tuesdays, Thursdays and Saturdays from 1 to 4 p.m. Admission is free. For information, 603-431-2538.