Barnhill began his professional career in Miami with the clowning Miami Giants and Ethiopian Clowns. Taking on the persona of "Impo" or "Skinny" he pleased crowds along the Clowns' barnstorming trail with his antics and with his masterful pitching. Moving on to the roster of the New York Cubans in 1941 he left clowning behind and quickly became one of the leading pitchers in the Negro National League.
He made his first appearance in the East-West all-star game in 1941, returning again in 1942 and 1943. On the strength of his exceptional fastball Barnhill continued as the ace of the New York Cubans staff through 1948 while compiling a .700+ win percentage.
Barnhill's chance to play in the major leagues was ultimately thwarted by his age. In 1949 he was signed by the New York Giants and assigned to their Triple-A farm team in Minneapolis. In 1950 he led the Minneapolis Millers pitching squad with an 11-3 record helping to propel the Millers to the American Association pennant. Despite his strong performance the Giants declined to elevate him to the major leagues citing his age, 35, as the reason.
Barnhill retired after the 1953 season.