In 1816, Tillinghast was admitted to the Rhode Island bar and opened his first office, in the village of Chepachet.
The following year, he returned to Providence and partnered with Samuel W. Bridgham, under whom he had studied at Brown University.
In 1843, Tillinghast represented Providence for a single term in the Rhode Island General Assembly.
In 2000, it merged with the Boston firm of Perkins Smith & Cohen and called its Rhode Island office Tillinghast Licht Perkins Smith & Cohen, but the firms split only a few years later, and Tillinghast Licht returned to its former name.
In May 2008, Tillinghast Licht announced that it will wind down business in the next few months, with six key lawyers joining Adler Pollock & Sheehan.