Born in London in 1935, Quennell earned his diploma in architecture from the Architectural Association in London in 1956 and then worked for architect Leonard Manasseh and for the housing division of the London County Council. In 1961 Quennell arrived in New York City, but soon settled in Cambridge, Massachusetts, where he worked for Josep Lluís Sert (of Sert, Jackson & Gourley). One year later Quennell joined the San Francisco firm of Lawrence Halprin & Associates and was assigned to work on Ghirardelli Square almost immediately. While at the firm, he flirted with the idea of being an artist and returned to New York City in 1967, living in the storied Chelsea Hotel. To support his artistic ambitions, Quennell took a job with Vollmer Associates and then earned his M.L.A. from Harvard’s Graduate School of Design in 1969. He then established his own landscape architecture practice before teaming with Peter Rothschild to found Quennell Rothschild Associates in 1979 (renamed Quennell Rothschild & Partners in 1998).
Quennell restored and rehabilitated many historic landscapes in New York City, including Fort Tryon Park, Sakura Park, Fort Greene Park, and Prospect Park. His notable collaborations with artists include the East River 60th Street Pavilion in New York City with Alice Aycock, Rosa Parks Circle in Grand Rapids, Michigan, with Maya Lin, and the Joseph M. Bryan, Jr., Theater at the North Carolina Museum of Art’s Museum Park with Barbara Kruger. He taught landscape architecture at the City College of New York and Columbia University, and from 1993 to 1997 was president of the New York City Art Commission. Quennell also served as co-chairman of the National Association for Olmsted Parks. He is now retired from practice and lives with his wife in New York City.