Treat Arnold earned his B.Arch from the University of Pennsylvania in 1957. There he met his future partner Robert Bishop who taught at Penn between 1952 and 1960. Arnold joined the firm of Montgomery & Bishop in 1962 and remained with the firm Montgomery, Bishop & Arnold until 1971, when he moved to New Hampshire and established a practice there.

Montgomery, Bishop & Arnold designed many significant projects. A notable design within Whitemarsh Township was the award-winning William Jeanes Memorial Library, built in 1971. Treat Arnold even had a hand in the design of the Wissahickon Ski Club lodge in Miquon in the late 1950s.