Josiah Cotton and Richard Fitzgerald, a teacher at a local Latin school, were responsible for young Cushing's early education.

He practiced law until 1772, when he was appointed by Governor Thomas Hutchinson to replace his father (who had resigned) on the Superior Court bench.

Not long after his tenure on the Massachusetts bench began, a controversy arose over revelations that court judges were to be paid by crown funds from London rather than by an appropriation of the provincial assembly.

After the American Revolutionary War broke out in April 1775, the Massachusetts Provincial Congress (which exercised de facto control over the province outside besieged Boston), sought to reorganize the courts to remove the trappings of British sovereignty.

In 1783, Cushing presided over a series of cases involving Quock Walker, a slave who filed a freedom suit based on the language of the new state constitution.

In Commonwealth v. Jennison, Cushing stated the following principles, in his charge to the jury: As to the doctrine of slavery and the right of Christians to hold Africans in perpetual servitude, and sell and treat them as we do our horses and cattle, that (it is true) has been heretofore countenanced by the Province Laws formerly, but nowhere is it expressly enacted or established.

[10][11] The case relied on a 1781 freedom suit brought by slave Elizabeth Freeman, also known as Mum Bett, on the same grounds; a Massachusetts county court ruled in her favor in 1781.

During Shays' Rebellion (1786–87), Cushing ensured that court sessions continued, despite the aggressive protests of the armed rebels, and later presided over their trials.

[12] On September 24, 1789, President George Washington nominated Cushing for one of the five associate justice positions on the newly established Supreme Court.

His two most important decisions were probably Chisholm v. Georgia and Ware v. Hylton, which held that treaties made under the Constitution supersede state law.

[14] While a justice, Cushing ran as the Federalist nominee in the 1794 Massachusetts gubernatorial election, losing in a landslide to incumbent Samuel Adams.