Harold Horner was born in 1911 on a farm near Creelman. He graduated from the University of Saskatchewan with a BSc in Agriculture in 1933 and an MSc in 1936, specializing in genetics and plant breeding. 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, where he worked on improving crested wheatgrass and sweet clover. Horner served in the Canadian army during World War II and was discharged in 1946 with the rank of captain. That same year he was appointed assistant director of agricultural representatives for Saskatchewan, and in 1947 became field crops commissioner.
In 1951 he was named Deputy Minister of Agriculture for Saskatchewan, and served in that role for twenty-one years. In that capacity he initiated the Family Farm Improvement Branch and the Economics and Statistics Branch. Horner was also instrumental in the development of the Veterinary Services Act, the Agricultural Development and Adjustment Act, and the South Saskatchewan Irrigation Act; and he oversaw policies relating to community pastures and conservation-earned assistance. He took an active role in the Gardiner Dam Irrigation Project, and the development of the Western College of Veterinary Medicine and the Saskatchewan Veterinary Diagnostic Laboratory. He served as chairman of the Prairie Agricultural Machinery Institute's three-province council from 1974 to 1980.
In 1972 the Saskatchewan government appointed Horner as executive advisor of Grain Handling and Transportation Systems Rationalization. In 1977 the Canada West Foundation appointed him chairman of a task force to produce a booklet, Western Canadian Agriculture to 1990, which was published in 1980. He received an honorary life membership in the Saskatchewan Agricultural Graduates Association in 1970, and in 1973 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. In 1992 Harold Horner was inducted into the Saskatchewan Agricultural Hall of Fame.