About twenty years later, a second attempt to form a new county was successful, and the Pennsylvania Assembly passed the act creating Lehigh County on March 6, 1812. The lots which became site of the old courthouse were sold to the three appointed County Commissioners by William Tilghman on November 19, 1812.

The first term of Lehigh County Court was opened on December 21, 1812. On that day, the Court of Common Pleas, of Oyer and Terminer and General Jail Delivery, and the Orphans' Court was opened under President Judge the Honorable Robert Porter, Esq. On that day, nineteen attorneys were admitted, there were nineteen civil cases, three petitions to open roads, and one case praying for the appointment of a Commissioner to examine a person of unsound mind. Three persons made declaration of their interest in becoming citizens.

The first term of court was held in the hotel of George Savitz on the main square in Allentown, which later became the Hotel Allen. In 1813, the court moved to the upper story of the prison building, which had been erected on the southeast corner of Margaret and Andrew Streets (later known as 5th and Linden Streets). Court was held here until final completion of the courthouse at 5th and Hamilton Streets. The plan of construction for the courthouse was secured by a citizen named John Yeakel, who had visited Lycoming County and had obtained a plan of the courthouse there. The total cost of completion was $24,937.08 and court was held in the newly built courthouse for the first time in August 1817.

The first President Judge of Lehigh County, Robert Porter, had also been a President Judge in Northampton County. He had been a member of the Fourth Pennsylvania Artillery Regiment near the end of the Revolutionary War at the grand old age of thirteen. He had been helped in his enlistment by his father, who was Colonel of the Regiment. After the war, he studied law and was admitted to practice on May 15, 1789. He was appointed to the position of President Judge in the newly created Lehigh County by Governor Simon Snyder.

The first major renovation of the courthouse took place in 1864, and, by the new century, the building was getting crowded. In 1915, a new wing was constructed to the rear of the building to add a second courtroom and office space.

By the 1960's, the old Courthouse could not accommodate an expanding County government and a new building was proposed. Designed as part of a government center next to Allentown City Hall, the newly-constructed eight story courthouse was dedicated at Fifth and Hamilton Streets in June 1965.

In a further expansion over thirty years later, County Administrative offices moved to a new building at 17 South 7th Street in 1997. The new Courthouse is currently being renovated for expansion of the Judiciary and court-related offices.