September 15, 1789, the Assembly adopted another resolution calling for a convention by a vote of 39 to 17.
At the election in October delegates were chosen, and on Tuesday, November 24, 1789, the convention assembled in Philadelphia, but a quorum not being present, the organization was effected the following day with sixty-four delegates in attendance. No returns had been received from the counties of Northumberland and Allegheny, and Mifflin had sent a double delegation.
Thomas Mifflin was chosen president; Joseph Redman, secretary; Frederick Snyder, messenger, and Joseph Fry, doorkeeper.
On the Republican side, those in favor of a new constitution were James Wilson, Thomas McKean and Thomas Mifflin, all of Philadelphia; Timothy Pickering, of Luzerne; Edward Hand, of Lancaster. Among the Constitutionalists were William Findley, of Westmoreland; John Smilie and Albert Gallatin, of Fayette; Robert Whitehill and William Irvine, of Cumberland.
After a long session the convention adjourned Friday, February 26, 1790, to meet Monday, August 9.
The second session of the convention met pursuant to adjournment and got down to business the third day, and concluded its work by the final adoption of a new instrument September 2, 1790, the final vote being sixty-one to one, Mr. George Roberts, of Philadelphia, voting against its adoption.
The most radical changes were made in the executive and legislative branches of government. The Assembly ceased to have the sole right to make laws, a Senate being created. The Supreme Executive Council was abolished. A Governor was directed to be elected to whom the administration of affairs was to be entrusted.
The former judicial system was continued, excepting that the judges of the higher courts were to be appointed during good behavior, instead of seven years. The Bill of Rights re-enacted the old Provincial provision copied into the first Constitution, respecting freedom of worship and the rights of conscience. The Council of Censors ceased to have authority and Pennsylvania conformed in all important matters to the system upon which the new Federal Government was to be administered.
The first election held under the Constitution of the Commonwealth, that of 1790, resulted in the choice of Thomas Mifflin, the president of the convention, which made, adopted and proclaimed the Constitution, for Governor. He served three terms.