From a strongly Calvinist family, he was educated at Leiden, He arrived in England as the University of Cambridge's first ever professor of History. From the outset he attacked the legitimacy of kings and justified revolt, as when he emphasized the Anglo-Saxon roots of England before 1066, emphasizing what he saw as democratic freedom enjoyed by all Englishmen before they lost it to the Norman conquerors and also justified the Dutch uprising against Spain. His apparent propagandizing for republicanism and regicide was seen as aimed at the King Charles I of England, who was suspected of Catholic sympathies and failure to uphold the country's interests against powerful foreign enemies. Despite his thinly veiled condemnation of the reigning royal power, little real action was taken against him apart from his doctorate being delayed. In 1629 he was admitted a commoner of the College of Advocates. In 1632 he made his peace at court, and on two occasions acted as judge advocate, in the Bishops War of 1640 and in 1642 in the army commanded by the Earl of Essex. Despite his early advocacy of freedom from untrammeled power, once ensconced in a position of authority, Dorislaus attempted to sweep away legal protection for the accused. He assisted in preparing the charge of high treason against Charles I, but was not prominent in the proceedings themselves.