Graham Saunders is a man in high demand.

When U.S. cannabis companies need financing they can’t find elsewhere, they turn to this Toronto banker who operates far from Wall Street. Since the spring of 2016, Mr. Saunders’s team at Canadian boutique firm Canaccord Genuity Group Inc. has helped finance more than half of all pot deals in the global equity markets, raising more than $5 billion from investors, according to Dealogic.

The...