Lewthwaite developed his box lacrosse skills in the Royal City’s Sapperton district, but by age 15 he was considered too big to continue playing at the juvenile level. Instead, he lined up with the New Westminster Junior B team that captured the 1966 Canadian championship in Port Arthur.
With the Senior A Salmonbellies in 1967—he was not yet 17 years old—he scored two goals. After the 1968 Minto Cup series, he played a pivotal role in the Salmonbellies National Lacrosse Association professional championship victory over Detroit. In 1969 Lewthwaite was 6’3”, 230-lbs, fleet of foot, a deadly shooter, a natural playmaker and a rib-crunching checker. At the age of 18, he became a permanent member of the Senior Salmonbellies, registering 87 points in his first 27 games.
Between 1970 and 1974, Lewthwaite and his teammates captured three Mann Cups in four trips to the Canadian championships. In 1975, the upstart pro National Lacrosse League team in Boston drafted Lewthwaite in the first round, but traded him to the Long Island Tomahawks where he accumulated 140 points in 47 games; however, he also seriously injured his knee.
After one season as co-coach of a Senior B team, he took over the coaching reins for the Salmonbellies in 1978. Over the next 21 years, Lewthwaite held similar positions with Coquitlam, Richmond, Burnaby and Maple Ridge.