Although he died relatively young with most of the fame going to his brothers, his extended McCormick family continued to be influential in the politics and business of Chicago.

[2] In 1850, William, his wife, and infant son moved to Chicago to join his brothers in the family business, first on a salary basis.

The California Gold Rush had created a labor shortage, which was good for demand, but also caused turnover in the factory employees.

William tried to mediate between Cyrus (who claimed all the credit as "inventor") and his brother Leander, who was now in charge of technical aspects of product development.

[3] In 1859, he negotiated a formal agreement giving him and Leander one fourth interest in the company, and renamed it "C. H. McCormick & Brothers.

Brother Cyrus worsened the conflict by funding conservative newspapers to battle the liberal major publishers that were tied to local politicians.

[5] Together, William and Mary were the parents of five children:[2] Outwardly, McCormick was described as "of a cheerful disposition, quick spoken, ... a most interesting and agreeable man".