After signing with Universitario de Deportes football club as a junior player in 1957 Tomás Iwasaki made his senior début for the team in 1959. Even though Iwasaki played for several domestic clubs in Peru during his career, his three national championship wins all came with Universitario, with back-to-back titles in 1959 and 1960, followed by a third title in 1967. He also made his début for the national team in 1959, playing in Peru’s 2–1 defeat to Paraguay in the South American Championship in Argentina. The following year Iwasaki was part of Peru’s squad for the football tournament at the 1960 Roma Olympics where he scored in their 3–1 win against India.