Complex sentence: He served on the City Council from 1802 to 1806 and then again in 1812, when Congress restructured city ordinances to create a council of aldermen for the city, which will then elect the mayor.
Split into simpler sentences: He served on the City Council from 1802 to 1806 and then again in 1812. At that time Congress restructured city ordinances to create a council of aldermen for the city, which in turn elected the mayor.