In 1943, when Stamos was 21 years old, prominent dealer Betty Parsons gave him a solo exhibition at her Wakefield Gallery and Bookshop. Parsons became an important ally and connection to the contemporary New York art world; Stamos would show regularly with her until 1957. By the mid-1940s, his career was becoming well established—he exhibited at the Whitney Museum annually from 1945 to 1951, at the Carnegie Institute and the Art Institute of Chicago in 1947, and at the Museum of Modern Art in 1948. Also during this period, Stamos’ work began attracting the attention of collectors. The Museum of Modern art purchased Stamos’ Sounds in the Rock in 1946. And Edward Wales Root, who became both a supporter of Stamos’ career and a benefactor of the Munson-Williams-Proctor Institute, bought the first of many paintings from the artist in 1945.

Stamos traveled widely during much of his adult life. In 1947, he traveled by train to New Mexico and the Pacific Northwest. In 1948 and 49, he visited Europe, including parts of Greece, and possibly Egypt. For the next four decades, Stamos traveled widely and frequently. These trips both contributed to his aesthetic development and also provided fodder for his broad, deep intellectual interest in the world’s belief systems. Beginning in 1962, he created several long series of paintings; many of these contained sub-series. The Sun-Box series, begun in 1962, explored hard-edged geometries on flat grounds. After 1971, all of his paintings were part of the Infinity Field series. These abstractions are characterized by broad areas of color delineated by slim lines or shapes; the effect is subtle and meditative. Among the Infinity Fields are the Lefkada sub-series, inspired by the Greek island where Stamos spent much of his time from 1970 until his death.

During the late 1940s he became a member of The Irascible Eighteen, a group of abstract painters who protested the Metropolitan Museum of Art’s policy towards American painting of the 1940s and who posed for a famous picture in 1950; members of the group considered as the ‘first generation’ of abstract expressionists included: Willem de Kooning, Adolph Gottlieb, Ad Reinhardt, Hedda Sterne, Richard Pousette-Dart, William Baziotes, Jimmy Ernst, Jackson Pollock, James Brooks, Clyfford Still, Robert Motherwell, Bradley Walker Tomlin, Theodoros Stamos, Barnett Newman, and Mark Rothko. These artists are part of the New York School and they were referred to as The Irascibles in an article featured in an issue of Life where the infamous Nina Leen photograph was published.

Around 1950, Stamos began exploring a new approach to abstraction. Inspired by East Asian aesthetics, he created his Tea House series of paintings, characterized by softly defined geometric forms painted with a limited palette and often overlaid by dark calligraphic brushwork. Later in the 1950s, Stamos worked with compositions that became increasingly reductive and simplified. He explored the use of layers of thin pigment, carefully worked, to create depth in his broad expanses of color.

A little over a year after his suicide in 1970, Rothko’s daughter sued the estate’s executors, as well as the Marlborough Gallery, for waste and fraud. Over twelve years of litigation and appeals, it was revealed that many of Rothko’s paintings, which had been sold or consigned by his estate to the Marlborough Gallery in Manhattan, were sold at intentionally deflated prices to favored clients while the gallery collected inflated commissions as high as 50 percent, compared with the 30 percent usually charged for an artist of his caliber; the executors, meanwhile, divided the estate’s proceeds from Marlborough as their fees. Stamos willingly joined the conspiracy, and was enticed to switch his representation from the André Emmerich Gallery by a more generous contract with the Marlborough. The defendants were found guilty and fined over $9 million; Stamos paid his share of the fine by signing over his house to the Rothko estate, but he was granted life tenancy. While the case did much to enhance Rothko’s reputation, it did serious damage to the reputation of both the gallery and Stamos. Another perspective on the case was published in the New York Law Journal in 1988.