Armour earned his first capital in California mining endeavours and cofounded a grain-dealing and meatpacking business in Milwaukee, Wisconsin, in 1863. Anticipating a sharp decline in pork prices near the end of the American Civil War (1861–65), he made nearly $2 million buying pork at depressed rates and selling it for far more in New York City. He became involved in his brother Herman Ossian Armour's grain-commission house in Milwaukee, to which he added a pork-packing plant in 1868. During the following decade the family's interests were concentrated in Chicago, and Philip Armour assumed leadership of the firm in 1875. He originated a number of slaughtering techniques, the use of waste products, and the sale of canned meat. When refrigeration was introduced in the 1880s, he established distributing plants in Eastern cities and began exporting meat products to Europe.

In his later years Armour employed part of his wealth, estimated at $50 million, for philanthropic purposes, founding the Armour Mission (opened in 1886 and funded in part by a bequest from his late brother Joseph F. Armour) and the Armour Institute of Technology (founded in 1890, later the Illinois Institute of Technology), both in Chicago. After his death, Armour's firm continued to prosper under the leadership of his son, Jonathan Ogden Armour, at whose retirement in 1923 the company was the largest meatpacking firm in the world. Although the company was sold in 1970, Armour remained a leading American brand of canned and cured meat products.