Hart was elected to the Hunterdon County
Board of Chosen Freeholders in 1750. He was first elected to the New Jersey Colonial Assembly in 1761 and served there until 1771. He was appointed to the local
Committee of safety and the
Committee of correspondence, and became a judge on the
Court of Common Pleas. He was often called "Honest John." When New Jersey formed a revolutionary assembly (or provincial congress) in 1776, he was elected to it and served as its vice president. Prior to June 1776, the New Jersey delegation in the
First Continental Congress was opposed to independence. As a result, the entire delegation was replaced, and Hart was one of those selected for the
Second Continental Congress. He joined in time to vote for and sign the Declaration of Independence. He served until August of that year, then was elected speaker of the newly formed
New Jersey General Assembly. He later took on additional duties as Treasurer of the Council of Safety (which was given "extraordinary and summary powers" to carry out affairs of the state during emergencies), President of the joint meetings of the New Jersey Congress, and Commissioner of the State Loan Office. ==Revolutionary War==