Patrick left home in 1871 and traveled to Dalton, Georgia, and the home of his grandfather, Duff Green. A relative defrayed the cost of his single year of high school education. He studied law thereafter under his grandfather's tutelage, and was admitted to the State Bar Association of Georgia. He moved the following year to St. Louis, Missouri, and was admitted to the Missouri bar. He suffered a severe health breakdown after a year, and went to live on the Arkansas farm of his brother, John C. Calhoun II. An attorney from Atlanta, Georgia, offered him a partnership in 1878 if he would move to Atlanta. This association was very brief, and he founded the law firm of Calhoun, King & Spalding. Focusing on corporate law, the firm became highly profitable. Calhoun used his profits to found the Calhoun Land Company (which invested in cotton production in the Mississippi Valley); buy extensive real estate in Georgia, South Carolina, and Texas; and invest or buy stock in manufacturing, mining, oil, and railroads.