After the war Marca-Relli joined the "Downtown Group" which represented group of artists who found studios in lower Manhattan in the area bounded by 8th and 12th street between First and Sixth Avenues during the late 1940s and early 1950s. During the late 1940s and early 1950s, he was actively involved in the avant-garde art world in Greenwich Village. These artists were called the "Downtown Group" as opposed to the "Uptown Group" established during the war at The Art of This Century Gallery.
His first one-man show was in New York City in 1948.In 1949 Marca-Relli was among the founders of the "Artists' Club" located at 39 East 8th Street. He was selected by his fellow artists to show in the Ninth Street Show held on May 21-June 10, 1951. The show was located at 60 East 9th Street on the first floor and the basement of a building which was about to be demolished.
The artists celebrated not only the appearance of the dealers, collectors and museum people on the 9th Street, and the consequent exposure of their work but they celebrated the creation and the strength of a living community of significant dimensions.
Conrad Marca-Relli was among the 24 out of a total 256 New York School artists included in the Ninth Street Show and in all the following New York Painting and Sculpture Annuals from 1953 to 1957.,,, These Annuals were important because the participants were chosen by the artists themselves.
Marca-Relli's early cityscapes, still lifes, circus themes and architectural motifs are reminiscent of Italian surrealist painter Giorgio de Chirico. Throughout his career, Marca-Relli created monumental-scale collages. He combined oil painting and collage, employing intense colors, broken surfaces and expressionistic spattering. He also experimented with metal and vinyl materials. Over the years the collages developed an abstract simplicity, evidenced by black or somber colors and rectangular shapes isolated against a neutral backdrop.
In 1967, the Whitney Museum of American Art gave him a retrospective show.