Harold Gretzner was born in Baltimore, MD.  He early training as an artist began in Washington D.C. but Gretzner did not complete his studies until he moved to the California. Upon moving to Oakland he took classes at California College of Arts and Crafts. With the onset of the Great Depression artists including Gretzner began exploring inexpensive media such as watercolor. No longer able to afford oil paint, artists like Milford Zornes, Rex Brandt, George Post, Millard Sheets, and Harold Gretzner began using watercolors. The quick-drying quality of the medium enabled artists to use controlled, yet expressionist brushstrokes.

Gretzner also explored printmaking. For most of his life, Gretzner worked as a commercial lithographer in San Francisco. He was a prolific painter as well, and exhibited both on the East Coast and in California. Gretzner was best known for his watercolors of urban life, cityscapes, landscapes, and seascapes.

As a member of the American Watercolor Society, California Water Color Society, West Coast Watercolor Society, and Society of Western Artists he was committed exclusively to watercolor painting. After retiring from commercial lithography Gretzner continued painting and taught a number of students in the Bay area.