Monboddo's early work in practising law found him in a landmark piece of litigation of his time, known as the Douglas "cause", or case. The matter involved the inheritance standing of a young heir, Archibald James Edward Douglas, 1st Baron Douglas, and took on the form of a mystery novel of the era, with a complex web of events spanning Scotland, France and England. Burnett, as the solicitor for the young Douglas heir, was victorious after years of legal battle and appeals.