After graduating from Brown University in 1816, he studied law and began a journalistic career in Providence, Rhode Island.
He joined and became a prominent member of the Suffolk County, Massachusetts bar.
In the latter race Charles Sumner was also a candidate, representing the Free-Soil Party.
In 1860 he was chosen as a delegate, but skipped the Charleston, South Carolina, meeting (the convention, scheduled April 23-May 3, 1860, coincided with the death of Hallett's wife, Laura Smith Larned, of bilious fever, on May 3, 1860).
He then joined the walk-out Convention that nominated John C. Breckinridge and Joseph Lane.