Margaret Harrington began her legal career in 1987, working for the United States Department of Justice in Washington, D.C. Her area of practice was environmental litigation involving the federal Clean Water Act and CERCLA, as well as an assortment of federal environmental regulatory compliance claims. Upon joining the Seattle law firm of Preston, Gates & Ellis in 1987, she expanded the scope of her environmental work to include a wide variety of state and municipal environmental regulatory compliance, toxic tort and environmental insurance coverage matters.

Ms. Harrington's areas of practice grew to include employment law while working as in-house counsel with Associated Industries of the Inland Northwest (AI), an association of approximately 200 diverse Spokane businesses. While with AI, she routinely advised member companies, through their human resource representatives, regarding corporate compliance with employment laws affecting all types of civil rights discrimination, wages and hours and corporate employee handbook drafting. In 2006, her insurance work grew to include plaintiff's ERISA claims as well as insurance defense arising out of automobile cases. She also practices in the area of plaintiff's personal injury.