Morris was born into a wealthy landowning family in what is now New York City. After attending King's College, now Columbia College, he studied law under Judge William Smith and earned admission to the bar. He was elected to the New York Provincial Congress before serving in the Continental Congress. After losing re-election to Congress, he moved to Philadelphia and became the assistant U.S. superintendent of finance. He represented Pennsylvania at the 1787 Constitutional Convention, where he advocated a strong central government. He served on the committee that wrote the final draft of the United States Constitution.