During his connection with the army, he had watched the course of affairs in his native State with the keenest interest, and in a series of four letters to the people of Pennsylvania, called their attention to the serious defects of their Constitution of 1776, the chief of which he declared to be the giving of the legislative power to one house only.  His appeals had the effect of bringing about an entire change in the form of State government, which was subsequently accomplished by a general convention of the people.  After the close of the war Dr. Rush was elected a member of the State Convention which ratified the Constitution of the United States, and distinguished himself in that body by his earnest and brilliant advocacy of that instrument.  He was also a member of the convention which adopted a new State Constitution, embodying the reforms he had advised in the letters referred to, and labored hard to have incorporated in it his views respecting a penal code and a public school system, both of which features he ably advocated through the public press.

With this closed his public career, which, though brief, was brilliant, and raised him to a proud place among the fathers of the Republic.

Returning to Philadelphia after resigning his position in the army, he resumed the practice of medicine, and with increased success.  His personal popularity and his great skill as a physician brought him all the employment he could desire, and he soon took his place at the head of the medical faculty of the country.

In 1785 he planned the Philadelphia Dispensary, the first institution of the kind in the United States, and to the close of his life remained its warm and energetic supporter.  In 1789 he was made Professor of the Theory and Practice of Medicine in the Philadelphia Medical College, and when that institution was merged in the University, in 1791, he was elected to the chair of the Institute and Clinical Medicine.  In 1797 he took the professorship of Clinical Practice also, as it was vacant, and was formally elected to it in 1805.  These three professorships he held until the day of his death, discharging the duties of each with characteristic brilliancy and fidelity.

The great professional triumph of his life occurred in the year 1793.  In that year the yellow fever broke out with great malignancy in Philadelphia, and raged violently for about one hundred days, from about the last of July until the first of November.  Nothing seemed capable of checking it.  The people fled in dismay from their homes, and the city seemed given over to desolation.  In the terrible “hundred days,” during which the fever prevailed, four thousand persons died, and the deaths occurred so rapidly that it was frequently impossible to bury the bodies for several days.  The physicians of the city, though they remained heroically at their posts, and labored indefatigably in their exertions to stay the plague, were powerless against it, and several of them were taken sick and died.  Few had any hope of checking the fever, and every one looked forward with eagerness to the approach of the season of frosts, as the only means of saving those that remained in the stricken city.