He graduated with a BSA degree at the University of Saskatchewan in 1933 and three years later obtained his masterís degree from the same university.

In July, 1937, he joined the staff of the Dominion Experimental Station at Scott, where he was in charge of soil erosion and snow conservation experiments. In 1939 he moved to the Dominion Forage Crops Laboratory in Saskatoon.

He served in the Canadian army during the Second World War and was discharged in 1946 with the rank of captain.

In 1946 he was appointed assistant director of agricultural representatives for Saskatchewan and in 1947 became field crops commissioner. Harold Horner was named deputy minister of agriculture for Saskatchewan in 1951 and served in that role for 21 years.

During his term as deputy minister, he was responsible for initiating the Family Farm Improvement Branch and the Economics and Statistics Branch. He developed and saw approved the Veterinary Services Act, the Agricultural Development and Adjustment Act, the South Saskatchewan Irrigation Act, and oversaw policies relating to community pastures and conservation earned assistance. He took an active role in the Gardiner Dam Irrigation Project, development of the Western College of Veterinary Medicine and the Saskatchewan Veterinary Diagnostic Laboratory. He served as chairman of the Prairie Agricultural Machinery Instituteís three-province council from 1974 to 1980.

In 1972 the Saskatchewan Government appointed Harold Horner executive advisor, Grain Handling and Transportation Systems Rationalization. He retired in 1976. In 1977 the Canada West Foundation appointed him chairman of a task force to produce a booklet, ìWestern Canadian Agriculture to 1990.î

His contributions were recognized in 1970 when he received an honorary life membership in the Saskatchewan Agricultural Graduates Association, and in 1973 when the Saskatchewan Institute of Agrologists presented him with its first Distinguished Agrologist Award. In 1984 the University of Saskatchewan conferred on him an honorary doctor of laws degree and in 1988 he received the Saskatchewan Order of Merit.