This, the county's third courthouse, dominates the borough of Mercer with its splendid dome. Though the firm of Owsley and Boucherle is credited with the design, the majority of the work was done by Charles F. Owsley, son of founding partner Charles Henry Owsley. Charles F. trained in New York, in Paris, and at the University of Pennsylvania (1903), after which he joined his father's firm in Youngstown in 1904, where he worked until his death in 1953.
The design met the county commissioner's stipulation that the courthouse be of red brick, rectangular, and trimmed with sandstone. The Beaux-Arts-inspired courthouse has matching Ionic porticoes on the front and rear elevations, with monumental staircases marking the entrances; the east and west entrances have smaller porticoes. The courthouse's finest feature is the ribbed dome that rises from a tall octagonal brick and stone base with paired columns on each elevation. On the interior, a central rotunda is flanked by two-story courtrooms. The rotunda's stained glass dome is highlighted with allegorical murals in the pendentives by Edward E. Simmons (1852–1931) entitled Innocence, Guilt, Justice, and Power, and stained glass panels above representing Peace, Truth, Law, and Justice. Public spaces have marble floors and wainscoting, brass railings, and modillion-lined ceilings.