Born in Buffalo, New York, Horgan was determined to be an artist from an early age. Along with acting, singing and writing talents, Horgan became a writer who painted. He wrote seventeen novels, plus biographies and histories. Although he never attended a college, he lectured at Yale and Wesleyan Universities and was awarded twenty honorary degrees.
In 1926 Horgan moved to New Mexico and continued to write, and earned money as a librarian at the New Mexico Military Institue in Roswell. He lived in New Mexico until 1960, when he moved to Connecticut. In New Mexico, he had close friends with Peter Hurd and Henriette Wyeth. Intrigued by the landscape of that area, Horgan wrote and illustrated a two volume epic titled "Great River" about the history of the Rio Grande. The book published in 1954, was one of the high marks of his career and won the Bancroft and Pulitzer prizes. In 1998, his publication, "A Writer's Eye: Field Notes and Watercolors," had watercolors from many places in the Southwest including New Mexico and Texas. His work conveyed a strong sense of place, especially of his beloved American Southwest.