Born in 1935 in Hampton Bays, New York, Hornbeck earned his B.A. at the University of Pennsylvania. He then studied at Harvard Graduate School of Design, where he received the Charles Eliot Traveling Fellowship in Landscape Architecture, the highest honor granted to an M.L.A. student by faculty members. After graduating in 1959 he founded Hornbeck Associates in North Andover, Massachusetts, and collaborated on numerous projects with influential practitioners and firms including Dan Kiley; Fletcher Steele; Sasaki, Dawson & DeMay; and Shepley Bulfinch Richardson & Abbott.
In 1963 Hornbeck joined the faculty at the Harvard Graduate School of Design, where he served as a professor of landscape architecture and city planning until 1980. Hornbeck’s research and teaching focused on historic landscape preservation, and he worked on master plans for the restoration of several prominent landscapes, including two Olmsted Brothers-designed estates: Castle Hill in Ipswich, Massachusetts, and Oldfields in Indianapolis, Indiana. In 1981 Hornbeck and his former student Susan Child founded the landscape architecture firm Child, Hornbeck Associates, Inc.
Upon his passing in 1998 due to cancer at the age of 62, Hornbeck bequeathed Broadfields, his Olmsted Brothers-designed, 14-acre home in North Andover, Massachusetts, to Harvard’s Department of Landscape Architecture. The funds garnered from the estate’s sale were used to endow the Hornbeck Professor-in-Practice of Landscape Architecture position, and to establish the Peter Louis Hornbeck Fund to support the Department of Landscape Architecture.