He initially practiced in Columbia County before moving to New York City. In 1838, New York Governor William L. Marcy appointed Morris Recorder of New York City, a position equivalent to a deputy mayor. He served in that capacity until 1841, when Governor William H. Seward removed him from office in connection with the Glentworth scandal.
The Glentworth conspiracy involved a plot by tobacco inspector James B. Glentworth to send workers from Pennsylvania to New York under the guise of laying pipes for the city, but in reality to cast votes for Whig Presidential candidate William Henry Harrison.
Morris, the district attorney, and then-Mayor Isaac Varian feared that documents essential to the ensuing grand jury would be destroyed, and so went personally to seize the documents. Governor Seward removed Morris from office for his actions.
Morris, a Democrat, became involved in the Tammany Hall political machine in the early 1840s. He was elected mayor in 1841 by a slim margin, and again in 1842 and 1843 by more substantial margins.
While serving as mayor in 1841, Morris took part in the investigation and arrest of John C. Colt for the murder of Samuel Adams.
In 1845 Morris was appointed Postmaster of New York City, and he served until 1849. In 1852 he became a justice of the New York Supreme Court. Morris died in New York City on October 24, 1855.
He served as an assistant to United States. Attorney James A. Hamilton and as a member of the New York State Assembly in 1833 and 1834.