In 1772 he was elected one of the two members of the Legislature from the City of Philadelphia, and was re-elected the following year, when he was the colleague of Franklin, then just returned from his mission to England. So conspicuous were his services in the Assembly, that when the appointment of delegates to the first Continental Congress came to be made, Mifflin was selected as one, and he occupied a position of commanding influence. “When the news,” says Dr. Rawle, his biographer, “of the battle of Lexington reached Philadelphia, a town meeting was called and the fellow citizens of Mifflin were delighted by his animated oratory.” None did more than he to arouse the populace to a sense of the danger which threatened. He did not only exhort, but he put in practice his pleading. When the troops were to be enlisted and drilled, Mifflin was among the foremost to train them, and was selected as a major in one of the earliest formed regiments. The patriot blood spilled at Lexington and Concord fired a martial spirit throughout America by which the bold leaders in every State were nerved to resist and resent those unprovoked assaults, and when Washington appeared at the camp in Boston as the Commander-in-Chief of the American armies, Mifflin was by his side. Recognizing his great personal popularity, the ease and dignity of his manners, breadth and soundness of his views, Washington placed Mifflin at the head of his military family. In the absence of, or at the retirement from the table of the chief it fell upon Mifflin to occupy his place and do the honors; and for this duty, by his social position at home and his foreign travel he was admirably fitted. Colonel Mifflin was the first person in America who officiated as aide-de-camp. When Washington, July, 1775, organized the entire army, the difficult position of quartermaster general was assigned to Mifflin. The duties were new and arduous. Everything was in chaos. Order had to be established and system inaugurated. On May 19, 1776, Congress appointed and commissioned Mifflin to be a brigadier general and he was given command of Pennsylvania troops. An assignment to the active field was much more to his liking than one at headquarters. Upon taking the field Mifflin was relieved as quartermaster general by General Stephen Moylan, who was ill suited to the difficult task of providing for an army where the authority for calling in supplies was little respected and the means of paying for them was rarely in hand; and not long after accepting the position he abandoned it. Congress called upon Mifflin to again assume the duties of quartermaster general and he reluctantly responded to the call of his country, deeming it a matter of duty.