Complex sentence: He was mayor of Philadelphia from October 21,1828, to April 15,1829, then served as United States Attorney for the eastern district of Pennsylvania from 1829 to 1831.
Split into simpler sentences: He was mayor of Philadelphia from October 21,1828 to April 15,1829. Mifflin was United States district attorney for the eastern district of Pennsylvania from 1829 to 1831.