Active in Boston politics, he represented the city in the provincial assembly from 1761 to its dissolution in 1774, serving as the lower house's speaker for most of those years.

Because of his role as speaker, his signature was affixed to many documents protesting British policies, leading officials in London to consider him a dangerous radical.

He engaged in extended communications with Benjamin Franklin who at times lobbied on behalf of the legislature's interests in London, seeking ways to reduce the rising tensions of the American Revolution.

An astute observer of the trade ties binding the various parts of the British Empire, he wrote in 1763 that extant high duties on molasses (payment of which was widely flouted by New England merchants) would have significantly harmed the economies of Britain, North America, and the West Indies, in part by diverting significant quantities of scarce hard currency to those duties.

[14] John Adams wrote of Cushing that "[He] is steady and constant and busy in the interest of liberty and the opposition, and is famed for secrecy and his talent for procuring intelligence.

In addition to official correspondence such as petitions, the two men exchanged opinions on political developments in their respective areas and were broadly in agreement on the need for moderation in dealing with the British government.

[17] Seeking to deflect colonial blame for matters away from the British government and onto the royal governor, Franklin forwarded these letters to Cushing, with specific instructions that they only be shown to a few people.

[19] In the letters, Hutchinson and Lieutenant Governor Andrew Oliver made highly inflammatory suggestions that colonial rights needed to be abridged.

[17] The resulting scandal inflamed political tensions on both sides of the Atlantic, prompted a petition from the assembly that Hutchinson be recalled, and brought about Franklin's resignation as colonial agent and conversion to a pro-independence view after he was publicly lambasted for his role in the affair.

[20] Cushing held the post of speaker until the assembly was dissolved in October 1774 by Governor and British Army General Thomas Gage, who succeeded Hutchinson.

As a result, orders issued to General Gage in April 1775 for the arrest of radical leaders included Cushing's name.

[26] The loss, which decisively handed pro-independence forces a majority in the Massachusetts delegation, propelled Cushing firmly into Hancock's camp.

[28] Hancock, the president of the Continental Congress, rewarded Cushing's loyalty by securing for him the appointment as a commissioner of marine affairs.

[32] This effort was rejected by the state's eligible voters for a variety of reason, although there continued to be calls from the towns demanding a new constitution.

[37] In early 1785, Hancock offered his resignation to the legislature, partly as a political maneuver (although he claimed to be suffering a particularly bad case of gout at the time).

The election was again undecided by the electorate; Bowdoin prevailed in the General Court's decision-making, and Cushing again won the lieutenant governor's office.