Complex sentence: He worked at Cleary, Gottlieb, Steen & Hamilton LLP as an associate and then joined Day Pitney as an associate in 1998 and became a partner in that firm in 2003.
Split into simpler sentences: He worked at Cleary, Gottlieb, Steen & Hamilton LLP as an associate and then joined Day Pitney LLP as an associate in 1998. He became a partner in that firm in 2003.