After obtaining his degree, Kopelk moved to Boston and became the landscape architect for Harvard University. His next position was with Walter Gropius' firm, "The Architects Collaborative (TAC)" in Cambridge, MA working on projects in Iraq and Kuwait.

William KopelkAfter moving to NYC, Kopelk began his work in architectural interiors for the corporate world, including projects for the WNET offices, Silverscreen Partners Headquarters, Mitsui and the Dewey Ballantine law offices. Publication of his NYC apartment on the cover of the New York Times Home section, provided many more opportunities to design domestic and showroom interiors. Later, joining a corporate re-imaging firm, Kopelk worked directly on projects in Tokyo and London.

In 1997, Kopelk was asked by clients to relocate to Palm Springs, CA., and be the site representative of their iconic Desert Kaufmann House, Richard Neutra, Architect, which had undergone complete restoration. While living there, he was commissioned to design one of the property's signature gardens. Through this experience, he became passionately involved in the emerging preservation efforts of Palm Springs, becoming the President of the Palm Springs Preservation Foundation (PSPS) for nine years, as well as establishing and also chairing, for three years, the very successful Palm Springs Modernism Week annual event.