In the years that followed, becoming assistant curator in 1944 and curator in 1946, he completely reshaped the Division of Graphic Arts, built the graphic arts collection, and helped create the postwar Washington art scene. His print exhibitions brought the work of S.W. Hayter, Josef Albers, Adja Yunkers, Louis Lozowick, Karl Schrag, José Guerrero, Louis Schanker, Werner Drewes and Boris Margo to Washington audiences.

Kainen’s own paintings from the 1940s illustrated a shift away from Social Realism toward abstract expressionism. He found inspiration in the Victorian skyline and architecture of the buildings surrounding his studio in Dupont Circle. He was one of the first abstract artists working in the city and produced abstract compositions of symbols and forms that resounded with both his physical surroundings and personal experiences.

In 1946, Kainen was included on Ad Reinhardt’s “Tree of Modern American Art,” a succinct map of the artists and movements defining art of the modern era. In 1947, the Washington Workshop Center for the Arts opened its art school, and Kainen served as a painting and printmaking teacher and guide to many important artists. He helped to make the workshop a magnet for new talent and instrumental in furthering the careers of artists such as Gene Davis, Morris Louis, and Alma Thomas.

In 1946, Kainen was included on Ad Reinhardt’s “Tree of Modern American Art,” a succinct map of the artists and movements defining art of the modern era. In 1947, the Washington Workshop Center for the Arts opened its art school, and Kainen served as a painting and printmaking teacher and guide to many important artists. He helped to make the workshop a magnet for new talent and instrumental in furthering the careers of artists such as Gene Davis, Morris Louis, and Alma Thomas.

In 1949, the Corcoran Gallery of Art held a retrospective of Kainen’s prints. That same year, his national loyalty was questioned and he was placed under investigation by the Civil Service Commission’s loyalty board because of the petitions he signed and articles he wrote for leftist causes as a young man in New York. As explained in the book Washington Art Matters,* “He watched as Congress pressured government agencies to fire anyone who was ‘not in the best interests of the department’… Kainen kept his job for two reasons–one, the attitude of superiors at the Smithsonian, people, he said, ‘accustomed to dealing with ideas,’ and two, an earlier commendation by J. Edgar Hoover himself….Kainen had earned the FBI’s gratitude by teaching printing methods to agency trainees, and Hoover had sent a letter saying, ‘You’ve done our country a great service.’ When the FBI rediscovered this bit of history, the visits stopped.”

Kainen was not cleared of formal charges until 1954. The strain of this period became evident in his vivid abstractions with titles like Exorcist (1952) and Unmoored #2 (1952). Kainen later remembered this as a period when “I begin with the aesthetic balancing of forms, but these psychological ghosts take over.”

Kenneth Noland organized Kainen’s first retrospective at Catholic University in 1952. Kainen later called the show, which included fifty to fifty-five paintings, one of the most important of his career. In the mid-1950s, Noland and Morris Louis established the basis for the Washington Color School. Because Kainen defined the fundamental concept of Washington Color Painting as staining, he never considered himself a member of the Washington Color School or a Color Field painter. Also, by that period, Kainen rejected the popularity of abstract expressionism for a return to the figure and shifted his focus to elegant figurative work.

In addition to pursuing his own art and curatorial duties for the Smithsonian Institution, as well as teaching and mentoring, Kainen engaged his literary side with research and writing. He published numerous books, articles, exhibition catalogue and book forewords. In 1956, he received a grant from the American Philosophical Society to conduct research in Europe on English woodcut artist John Baptist Jackson. This resulted in his acclaimed monograph John Baptist Jackson: 18th-Century Master of the Color Woodcut. He traveled to Europe again in 1962 to study paintings and prints from the Mannerist Period. His book The Etchings of Canaletto was published in 1967. He also wrote poetry throughout his life, but declined to have it published despite urging from family and friends.

By 1966, Kainen was tired of his dual careers as an artist and curator and attempted to retire from the Smithsonian. David Scott, director of the National Collection of Fine Arts (now the Smithsonian American Art Museum), persuaded him to become a part-time curator of prints and drawings at his museum. Kainen’s so-called part-time position stretched into long hours and between 1966 and 1970, he increased the collection from 1,000 to 7,000 works.

Kainen and his wife Bertha divorced in 1968. In February 1969, Kainen married writer and art collector Ruth Cole. Their home in Chevy Chase, Maryland became a gathering place for the Washington art world, and Kainen often entertained guests by reciting from memory passages from his favorite writers, including Keats, Yeats, Milton, and Melville.

In their 32 years together, the Kainens built one of the finest collections of 20th-century German expressionist prints and drawings in private hands, as well as outstanding works by 16th-century Dutch mannerists and 20th-century American abstract expressionist artists. They also became major benefactors of the National Gallery of Art, Baltimore Museum of Art, the Jewish Museum New York, and other museums.

Kainen retired from the Smithsonian Institution in 1970 to devote himself full-time to his art, but continued to serve as a special consultant to the Smithsonian American Art Museum for nineteen years. In 1971-1972, Kainen taught painting and the history of printmaking at the University of Maryland.