On April 15, 1691, Jamison was appointed Deputy Secretary and Clerk of the council and began his legal studies. He then served as Clerk of the Court and was part of the group, along with Nicholas Bayard (nephew of Peter Stuyvesant), that tried Jacob Leisler (best known for Leisler's Rebellion following the English Revolution of 1688). In April 1698, when the Earl of Bellomont arrived in New York as Governor, Jamison was dismissed as Clerk. By October 6, 1698, Jamison was admitted to the New York bar and was an active member of the New York Bar Association (which was formed in 1709).