In the provincial election of 2003, Rinaldi defeated incumbent Progressive Conservative Doug Galt by about 2500 votes to become the new Member of Provincial Parliament (MPP) for Northumberland. The Liberals won the election, and Rinaldi was named Parliamentary Assistant (PA) to David Caplan, the Minister of Infrastructure, on October 23, 2003. He was re-elected in the provincial election of 2007 in the newly redistributed riding of Northumberland—Quinte West. He was appointed as PA to the Minister of Municipal Affairs and Housing.