Duncan has had an illustrious legal career as a solicitor, advocate, author and expert witness. He joined Slade Manwaring & Co in 1983 where he went on to be admitted as an equity partner and to establish their family law practice. In 2019 Duncan and Laura  founded the firm now known as Holmes Donnelly & Co Solicitors.

Duncan has held multiple professional memberships and appointments for the Law Society including sitting on the Family Law Committee (2004 – 2009), acting as an investigator for s.55 (LPA) matters (1991 – 2004), holding specialist accreditation in Family Law since the inception of the Accreditation Scheme in New South Wales (1983) and being on the Legal Aid Review Committee in Family Law (1993 – 2010). In 1998 Duncan commenced taking instructions from the Office of Legal Services Commissioner to review determinations of decisions regarding complaints against solicitors. Duncan has also been requested to act as expert witness, giving opinion evidence in the Family Court of Australia in costs and conduct issues as between solicitor and client, and giving opinion evidence in negligence matters involving the conduct of family law solicitors in the District and Supreme courts of New South Wales and the Supreme Court of the Australian Capital Territory.

In 1994 Duncan was selected to be on the Legal Aid Commission’s panel of solicitors to act as “separate representative” in children’s cases (now known as “Independent Children’s Lawyer” or ICL). With his extensive experience and expertise, he has appeared as ICL in various significant and complex parenting matters and reported cases. Duncan is an assertive and analytical advocate who is equally comfortable with written and oral submissions.

Duncan commenced writing for legal publishing house, Butterworths (now Lexis Nexis) in the mid-1980s, and in addition to acting as an editor and head noter, he has continuously contributed to their case books on Family Law and Torts in their “Student’s Companions” series. Duncan is regularly invited to lecture and chair Legal Education Programs and Continuing Legal Education seminars for the profession and to present at training sessions at the College of Law for those undertaking the Family Law Specialist Accreditation program.