Juhani Lahtinen initially started his sports career as a track and field athlete and became the Finnish junior champion in both the pole vault and the 4 x 100 metres relay in 1957. Lahtinen also began playing ice hockey around this time, with this becoming his sport of choice. He joined Tampereen Ilves in 1955 as a goal tender and remained at the club for the rest of his career. By the time he retired in 1970 he had played in 252 games for the team, winning five Finnish titles (1957–58, 1960–61, 1966) and twice being named the player of the year (1961, 1964). Lahtinen also played in 160 matches for the national team, winning silver at the 1962 European Ice Hockey Championships and competing at the 1960 Squaw Valley and 1964 Innsbruck Olympics.