Emil Kosa, Jr. received art instruction and music lessons at a very early age. When he was in his late teens, he had to decide between being a professional musician or artist. He chose art, but his family and instructors believed he could have been a famous musician had he chosen that occupation.

In the 1920s, he moved to California, but returned to France several times to continue his art education. He received traditional painting instruction from Pierre Laurens and studied non-objective painting with Frank Kupka. After settling in California in 1928, he worked as a mural artist and operated a business with his father that produced decorative art for churches and auditoriums. When that line of work was Slow, he took on portrait commissions and sold fine art paintings through local galleries.

In the early 1930s, Kosa became friends with Millard Sheets and with Sheet's encouragement, began aggressively pursuing a national reputation as a California watercolor artist. He sent up to sixty watercolors every year to museum shows all over America and was among the first California Style watercolorist whose work brought attention to the West Coast watercolor style. He was an active member of the California Water Color Society and served as president in 1945. Kosa was one of the first of the California watercolorists to be accepted into annual shows in New York City at the National Academy of Design and in the American Watercolor Society shows.

To financially support his family, Kosa worked as a scenic artist in the special effects division at Twentieth Century-Fox Studios for thirty-five years. He produced art for matt shots and was known as a top artist in this field. The motion picture industry acknowledged his contributions and awarded him an Oscar for his special effects work for Cleopatra.

A compulsive painter, Kosa would often paint for three or four hours after dinner each night and spend most of his weekends outdoors, painting with watercolors or oils. Alexander Cowie was his Los Angeles agent and the Macbeth Gallery sold his work in New York City. The Cowie Gallery, located in the Biltmore Hotel, had several one-man shows every year and included his work in all of their group showings.

Through his studio connections, he also produced a large number of commissioned portraits for movie stars, businessmen and politicians. In the 1950s, he was known as the premier portrait painter in Southern California. His official portrait of Earl Warren from this era, is in the collection of the National Portrait Gallery in Washington, D.C.

Kosa is best known for his representational watercolors and oils, but also won awards for pencil drawings and pastels depicting figurative subjects and prints. During the 1940s and through the mid-1960s, he occasionally revisited his interest in non-objective art and produced a body of work which expresses his love for music and experimental art concepts.