Lehmann has worked as a solicitor in his own small law firm, as an academic lawyer at the University of New South Wales, and as a corporate tax lawyer, having retired from PricewaterhouseCoopers.
Throughout his career, Lehmann has held various positions related to law. He started his own small law firm, worked as an academic lawyer at the University of New South Wales, and later specialized in corporate tax law before retiring from PricewaterhouseCoopers.