As an enthusiastic plantsman, Barrett began designing gardens in 1869 with only the practical experience gleaned from the family’s nursery business. His plan for a small garden in Bergen Point, New Jersey, led to a project for the New Jersey Central Railroad, and he soon became engaged in a collaboration with Chicago architect Solon Beman for a model town in Illinois envisioned by their patron, George Pullman. This community design led to project involvement in Chevy Chase, Maryland; Fort Worth, Texas; and Birmingham, Alabama. His design for Chevy Chase, of which certain elements remain, took inspiration from formal European precedents. Barrett was also involved in the design of many Country Place estates, including that of Naumkeag in Stockbridge, Massachusetts, for Joseph and Mabel Choate. Later revisions by Fletcher Steele honor the original spatial organization and concept put forth by Barrett.
In 1895, Barrett was appointed landscape architect of the Essex County (New Jersey) Park Commission, where he designed parks and the layout of boulevards. In 1903, he was elected president of the recently formed American Society of Landscape Architects. Barrett was an outspoken advocate for the field of landscape architecture. His own garden, an eclectic collection of a wide variety of garden types, from Colonial and English to Moorish, Roman, and Japanese, expressed an historical curiosity and creative energy far beyond the formality of his public work.