His interest in golf course architecture was aroused upon graduation in 1935, when he was hired to evaluate soils and find topsoil on the Capilano Golf Club, then under construction in West Vancouver, for Canadian architect Stanley Thompson.
Cornish then continued his training for four years with Thompson before becoming Head Greenkeeper at St. Charles Country Club, Winnipeg.
This was followed by a five-year association with pioneer turf grass scientist Lawrence S. Dickinson at the University of Massachusetts Amherst.
During his first years as a designer, Cornish was assisted in artwork and drafting by his wife, the former Carol Burr Gawthrop.
He soon established himself as a competent designer, and in 1964 took on a partner, young Penn State graduate William G. Robinson.