Mack was a clerk at his arrival in 1820, and on October 20, 1823 he received his first licence as a trader followed by licences on September 6, 1824 and October 5, 1826. Evidence suggests that Mack spent his winters at Grand Detour and then lived in Chicago during the summer. It is known that he voted occasionally and served as a clerk in elections there from 1828 to 1830. On September 29, 1830 he bought two lots in Chicago in the block bound by Randolph, Market, Washington and Water Streets.