Sibley was instrumental in passing the legislation in 1802 by which Detroit was incorporated as a town. Sibley was elected first as Chairman of the Board of Trustees, and then under the first city charter of 1806 as the first mayor of Detroit. During the War of 1812, Sibley commanded a company of riflemen in defense of Detroit, though the British attack was successful and William Hull surrendered the fort. After the war, Sibley served as Auditor of Public Accounts for the Michigan Territory from 1814 to 1817.