He was granted a 400 acres (162 ha) tract of land in Mississauga, Ontario (which at the time was undeveloped rural property) by the Crown, provided he built a home on it within four years.
Cawthra served one term as the Alderman for St. Lawrence Ward on the inaugural 1834 Toronto City Council before being defeated by conservative candidates.
Joseph's spell of municipal office, his sole venture into electoral politics, reflected not only his prominence in reform circles but also his active engagement in civic affairs.
In evidence given to the House of Assembly's select committee on the state of the provincial currency in 1830, he decried its privileged position as the only chartered bank in the colony and advocated a more competitive financial market.
Later that year, however, he acquired his first shares in the bank, possibly in order to stand as an "anti-establishment" candidate for its directorate, along with Jesse Ketchum, Thomas David Morrison, and Robert Baldwin.