Following graduation from the Ontario Agricultural College, Jack McCague served as Agricultural Representative, first in South Simcoe, and later in Carleton. In both counties he was very active in promoting the use of purebred livestock and certified seed potatoes. The results of his efforts are still apparent in those areas.

In 1928, he resigned from government service to become manager of Stevenson Farms at Alliston and proceeded to develop outstanding herds of Holsteins, Shorthorns, Yorkshires, and silver foxes.

Following the death of Mr. T.P. Loblaw in 1933, he purchased part of Stevenson Farms and named his holdings, Glenafton Farms. During the next 25 years he developed one of the most famous Holstein herds in North America. Animals produced on Glenafton Farms were sold to all parts of the world and, deservedly, the owner was named a Master Breeder.

At various times during his career he served as chairman of the National Live Stock Records; president of the Canadian Swine Breeders’ Association, the Canadian Silver Fox Breeders’ Association, The Holstein-Friesian Association of Canada, and the Dairy Farmers of Canada. In addition he was a director of the Canadian National Exhibition; and the first farmer to become a director of a Canadian chartered bank - the Toronto-Dominion Bank. Characteristically, he played a prominent role in formulating the policies of all organizations with which he was associated.