As a result of his distinguished service, in 1748 Colonel James was appointed Attorney General of the province.

Otis expected to be appointed Chief Justice of the Massachusetts Supreme Judicial Court, but the position instead went to Thomas Hutchinson appointed in 1761 by Governor Sir Francis Bernard, 1st Baronet; creating enmity between the Otis and Hutchinson families.

Otis was the presiding justice of the Barnstable County Court of Common Pleas during the Sept. 27, 1774, protest against the British "Intolerable Acts."

In meeting the protesters demands, he agreed to ignore the requirements of the Parliament's new legislation and so preserved for Barnstable the large measure of self-government that Massachusetts had enjoyed under its 1691 charter.

His son James Otis Jr. played a key role in opposing the British writs of assistance in 1761, serving to inspire the idea of revolution in the colonies.