The demolition of the first jail provided a site for a new courthouse, designed in the modern classical manner of LeBrun's teacher Thomas Ustick Walter's Chester County courthouse (
CH6). Dominated by an Ionic colonnade and portico in the broad proportions of the Greek Revival, the courthouse is built of Montgomery County blue marble. It was the third in line of local courthouses that departed from the small-scale eighteenth-century models, following Chester County (1846) and Delaware (1849; DE16) by Samuel Sloan. Over its century and a half of service, the building has been considerably altered. LeBrun's tall clock spire, which would have looked very much like Walter's spire at West Chester, was replaced with a cupola designed in 1902 by Schermerhorn and Reinhold, successors to Stephen D. Button's office, who also added the Baroque wings. The rear annex in the same blue marble is by Rankin and Kellogg, who spread civic classicism across eastern Pennsylvania. The courthouse was extended to the west with an underground wing by the Kling firm in their best corporate granite manner.