Complex sentence: Williamson joined the Labour Party in 1976 and became a councillor in 1991, then became Leader of the Labour Group on Derby City Council, serving as Leader of Derby City Council on two separate occasions.
Split into simpler sentences: Williamson joined the Labour Party in 1976 and became a councillor in 1991, then became Leader of the Labour Group on Derby City Council. And served as Leader of Derby City Council on two separate occasions.