Complex sentence: Williams was elected to the Fifteenth Congress and held office from March 4,1817 to March 3,1819; he was then elected to the Eighteenth Congress, holding office from March 4,1823 to March 3,1825.
Split into simpler sentences: Williams was elected again, to the 15th United States Congress, holding office from March 4,1817 to March 3,1819. He was then elected again, to the 18th United States Congress, holding office from March 4,1823, to March 3,1825.