John Zimmer founded the firm in Parker, South Dakota, in 1941. Dennis Duncan joined the firm in 1974, followed by Drew Duncan in 2003, and Amanda Engel in 2013. The firm continues to maintain its original office in Parker, South Dakota, but has also expanded to Sioux Falls, South Dakota.

From 1941 to today, The Duncan Law Firm, LLP, continues to uphold the firm’s founding principles of providing knowledgeable and efficient legal services to individuals and businesses in the State of South Dakota. The firm's state-wide areas of practice include governmental relations (lobbying), litigation, health care contracting, probate, domestic relations, and real estate transactions.

Our senior partner, Dennis Duncan, has been recognized with an AV Rating by Lexis Nexis and Super Lawyers (lobbying) by Thomson Reuters.