William Hagen has performed as soloist, recitalist, and chamber musician across the United States, Europe, and Asia. In 2021, William made his debuts with the Chicago Symphony Orchestra at the Ravinia Festival and the Chamber Orchestra of Europe at the Rheingau Music Festival, and appeared at the Santa Fe Chamber Music Festival. As soloist, William has appeared with the Detroit Symphony, San Francisco Symphony, Seattle Symphony, and regularly appears as soloist at the Aspen Music Festival.

In Europe, he has soloed with the Frankfurt Radio Symphony (HR Sinfonieorchester), the Vienna Radio Symphony (ORF Radio Sinfonieorchester Wien), and the major orchestras of Belgium, including the Brussels Philharmonic, National Orchestra of Belgium, and the Orchestre Philharmonique Royal de Liège. William has also soloed in Japan with the Yokohama Sinfonietta and the Sendai Philharmonic. As recitalist and chamber musician, William has performed at venues such as Wigmore Hall and the Louvre, and collaborated with artists such as Steven Isserlis, Gidon Kremer, Edgar Meyer, and Tabea Zimmerman, among others. He maintains an active schedule on both sides of the Atlantic, making frequent trips to Europe and cities around the US to play a wide range of repertoire.