As long as the US has existed, there has been corporate personhood. The Supreme Court has argued it in terms of corporations being essentially groups of people acting together for a common cause, and noted that working together should not invalidate the rights to free speech, religious practice, entering contracts, etc. Chief Justice Marshall said it this way in 1830: "The great object of an incorporation is to bestow the character and properties of individuality on a collective and changing body of men."

Key cases (you can find them all at http://supreme.justia.com/ ): Dartmouth College v. Woodward, 17 U.S. 518 (1819) Providence Bank v. Billings, 29 U.S. 514 (1830) Santa Clara County v Southern Pacific Railroad Company, 118 U.S. 394 (1886) United States v. United Auto Workers, 352 U.S. 567 (1957)