One of her daughters, Julia Fendrick, confirmed the death but did not give a specific cause.

Through her namesake gallery, Ms. Fendrick organized the first Washington solo shows of artists including Robert Arneson, Jim Dine, Helen Frankenthaler, Louise Nevelson and Johns, the protean painter, sculptor and printmaker whom she described as her “good-luck artist.” Twenty-one of his etchings and lithographs were featured at the Fendrick Gallery’s opening, in 1970; nearly 20 years later, they were taken out of Ms. Fendrick’s personal collection and used to open the gallery’s short-lived New York branch, in SoHo.

An energetic dealer with an expansive, at times unorthodox approach to art, Ms. Fendrick began selling drawings and prints in 1960 at the suggestion of her husband, Daniel Fendrick, a research official at the State Department. They had four young children, with a fifth to come. Her husband saw a need for more Washington art dealers, and he also was looking for a way to find Ms. Fendrick work outside the home.

He “felt that I was overwhelmed with small children in diapers,” she recalled in a 2007 oral history for the organization ArtTable, “and I better do something.”

Ms. Fendrick had no formal credentials, although she got what she considered a free education in art while working as a docent at the National Gallery. She also had help from a well-connected cousin, the painter and sculptor Irena Baruch Wiley, who introduced her to a friend who was trying to offload a collection of European prints.

Before long, Ms. Fendrick was traveling the country, selling modernist works by Braque, Picasso and Miró and expanding into American art with prints by Josef Albers, Robert Motherwell, Frank Stella and Roy Lichtenstein. She hosted clients at her home, which was filled with antique French furniture and framed prints, as well as a makeshift gallery of art by her children. (“Look at this — it was definitely influenced by a Rouault print we had here,” she told a Washington Post reporter in 1962. “And this one has something of a Picasso we have upstairs.”)

As Ms. Fendrick told it, she opened her gallery in Georgetown after her children got tired of having to finish dinner early and run upstairs, “out of sight, out of mind,” so that she could meet with curators and other clients. But the business remained a family affair: Her husband, a co-owner of the gallery, wrote its monthly newsletter and designed its plexiglass display frames, while her children helped out at openings and receptions, cleaning and cooking and occasionally serving champagne.

Under her direction, the three-story gallery became a showcase for Washington artists such as Sam Gilliam, John Grazier and Andrea Way. It also featured the work of sculptors such as Albert Paley, who gained national recognition for the decorative “Portal Gates” he created for the Renwick Gallery, and spotlighted artists whose mediums were seldom represented in major galleries or museums, such as glass artist Dale Chihuly and furniture makers Wendell Castle and Arthur Cotton Moore.

“I like to do one offbeat show every year,” Ms. Fendrick told the magazine Craft Horizons, looking back on shows such as “The Book as Art,” from 1976, which explored the intersection between writing and painting. The show included work by John Cage, Marcel Duchamp and Andy Warhol, and followed on the success of another unconventional show, “Clay USA,” which the Smithsonian’s Archives of American Art deemed “the first major show of contemporary ceramics on the East Coast.”

Asked about her philosophy as a gallerist, Ms. Fendrick said that her mission was simple: “To show the best.” Yet she also suggested that with shows built around mediums such as metal, clay and wood, she had grander ambitions as well.

“I was trying to point out,” she said in the oral history, “that objects can be as interesting and as worthy as a painting.”

The younger of two children, Barbara Johnson Cooper was born in Indianapolis on Dec. 18, 1929. Her mother was a homemaker, and her father was a Navy lawyer whose job led the family to move to Florida and Washington, where she graduated from Georgetown Visitation Preparatory School.

At 17, she went to Lisbon to live with her cousin John Cooper Wiley, the U.S. ambassador to Portugal, and his wife, Irena, the artist who helped her launch her career as a dealer. Ms. Fendrick affectionately described them as her aunt and uncle and accompanied the couple to their next diplomatic posting, in Tehran.

Irena “taught me how to look,” she said, by taking her to European churches and art museums. Years later, she agreed to be represented by Ms. Fendrick’s gallery.

Ms. Fendrick attended Georgetown University and met her husband when they were both enrolled at the Paris Institute of Political Studies, known as Sciences Po. They married in 1953 and settled in Chevy Chase a few years later, where Ms. Fendrick began selling art while drawing inspiration from two other prominent women in the art world: Alice Denney, a Washington impresario of the avant-garde, and Tatyana Grosman, a printmaker and publisher who encouraged her interest in fine-art prints.

In addition to her daughter Julia, survivors include four other children, Lila, Peter, Anne-Marie and John Fendrick, and 10 grandchildren. Her husband died in 1992, the year after Ms. Fendrick closed her galleries in Washington and New York amid an economic downturn.

Business was bad, she said, but she was also exhausted by putting on monthly shows. She turned instead to working as an art consultant, appraiser, lecturer and guest curator. “I’m always urging people to please go out and look,” she said in the oral history, “because there’s always something there.”