Mordi Gassner, painter, muralist, designer, and printmaker, was born Mordeca Gassner in New York City on May 27, 1899. He was an autodidactic artist who learned about art and sciences at the New York Public Library before attending the Brooklyn Institute of Arts and Sciences (later Parsons School of Design) from 1916 to 1919. He also studied at the Art Students League, where he taught painting and technique in the late 1950s. Building a reputation as a designer, Gassner was hired in the early 1920s by Hollywood executives to design film sets and among his credits is the Mary Pickford and Douglas Fairbanks film The Thief of Baghdad.

  
In 1928 and 1930, Gassner was awarded Guggenheim Fellowships to study in Florence, Italy. While living in Florence, he executed a mural project titled The Mural Monument to Modern Culture, which included several 9-foot-by-6-foot panels individually dedicated to biology, chemistry, physics, geology, the poet Petrarch, archeology/anthropology, and astronomy. The cartoons for the murals were exhibited in their entirety at the Brooklyn Museum in 1932, and later sections were shown at the Corcoran Gallery of Art in Washington, D.C.; the Academy of Art in in Richmond, Virginia, and the Museum of Modern Art, New York. Five panels were installed at the Dibner Building, Polytechnic University, in Brooklyn, New York.

In the years leading up to the Second World War, Gassner was hired by the WPA to design and execute several murals for the United States Postal Service as well as the Long Island Court House. In 1936, he created a series of lithographs of animals in the wild that were published by Associated American Artists. During the war, Gassner designed manuals for the Army Signal Corp. 

The second phase of Gassner's career was divided between teaching and stage and costume design. He worked for Broadway, television and opera productions. Beginning in the 1950s, he taught art history, painting, and scenic design at the New School of Social Research (now called The New School, merged with Parsons) and the Art Students League, and he continued to teach through the 1960s. His career as a designer continued through the 1970s.

Gassner’s work is represented in the National Gallery of Art, Washington, D.C. An interview with Mordi Gassner can be found at the Smithsonian archives online collection.

Mordi Gassner died in Drake's Beach, Virginia, on January 9, 1995.