Wittrup, an illustrator and portrait artist, was born John Shilling in Chicago. He began his artistic career at age eleven when he began selling editorial cartoons to local newspapers in his hometown. Wittrup was educated at the Chicago Art Institute and the American Academy of Art.
To establish himself as an artist, he held solo exhibitions throughout the United States. In 1931, he won honorable mention in an exhibit at the Wisconsin State Historical Society. During World War II, he served in the Army, working as an artist in New Mexico on propaganda posters; by the end of the war, he had attained the rank of sergeant.
After the war, Wittrup married Beatrice West and moved to New York City, where he earned a living as an artist and illustrator. He produced ads for various products including Lucky Strike cigarettes. Some of these advertisements are now collectibles.
Eventually, Wittrup moved to Boca Raton, but maintained a studio in New York. He was instrumental in establishing the Armory School and Visual Arts Center in West Palm Beach, now known as the Armory Art Center, and had taught at its forerunner, the Art Annex School of West Palm Beach.
During his career, Wittrup was commended by the Florida state senate for his work. He received commissions to paint five Florida senate presidents. Other examples of his commissioned work include Mary Kay, the founder of the cosmetics company, Walter Beardsley, the former chairman of Miles Laboratories, and Army Brigadier General George W. Goddard.