Upon his call to the bar, Henderson joined the firm of Henderson & Herridge (which later became Gowling Lafleur Henderson). Within 3 years, and before the age of 28, Henderson had appeared successfully on two separate occasions at the Supreme Court of Canada. Throughout his career, Henderson developed a reputation for litigation in all areas of law, especially in intellectual property. Ian Scott, former Attorney General of Ontario, called him "the best all-round lawyer the profession has produced since the War." By the time of his death, Henderson appeared as counsel in nearly 400 reported cases, including 90 before the Supreme Court of Canada.