William Cassils was born on June 25, 1832, in Denny, Scotland.

He was a telegraph operator and businessman. Educated in Renton, Scotland, he came to Canada in 1851 where he became an apprentice telegraph operator for the Montreal Telegraph Company. He advanced rapidly and soon was supervising the installation of a line following the Grand Trunk Railway. In 1853, he was promoted manager of the firm’s Quebec City office. After the absorption of Quebec's rival British North American Electric Telegraph Association in 1856, Cassils served as the superintendent of Montreal Telegraph’s Eastern division. From 1866 to 1876, he formed a partnership with George A. Cameron, opening a hoop-skirt factory and later evolving into a wholesale business for fancy dry goods in Montreal. In 1876, he left Cameron to organize the Canadian District Telegraph Company, of which he became president. About 1877 Cassils became president of the Canada Central Railway Company, which merged with the Canadian Pacific Railway in 1881. He then served as receiver of the financially floundering St. Lawrence and Ottawa Railway Company and president of the Dominion Transport Company. In 1889, he became president of the Federal Telephone Company. He was also president of the Electro-Mechanical Clock Company, vice-president of the British American Ranch Company, and director of the Montreal Herald, the Windsor Hotel Company of Montreal, and several financial institutions. He was also a secretary-treasurer of the Quebec Protestant School Board and a member of the St. James Club and the Fish and Game Club.

In 1856, he married Agnes Simpson Hossack (1840-1908). He died on December 25, 1891, in Montreal, Quebec.