Born in Exeter, New Hampshire, he attended Phillips Exeter Academy before establishing a legal practice in Zanesville, Ohio. After serving in the Ohio House of Representatives, he was appointed as a U.S. Marshal. Cass also joined the Freemasons and would eventually co-found the Grand Lodge of Michigan. He fought at the Battle of the Thames in the War of 1812 and was appointed to govern Michigan Territory in 1813. He negotiated treaties with Native Americans to open land for American settlement and led a survey expedition into the northwest part of the territory.