Alfred J. Hennessey was born in Charlottetown, Prince Edward Island in 1930, the son of William James Hennessey and Gertrude Coady. He graduated from the University of Manitoba in 1956 with a Bachelor of Architecture and obtained M. Arch. The following year from the Pratt Institute of Design. He began his practice in Chrlottetown in 1958 and continued to practice his profession until 1985 under the various names: A. J. Hennessey, Architect (1958, 1963-1970); Architects Associates (1959-1963); Hennessey Watson Associates (1970-1980) and Hennessey Consultants Incorporated (1980-1985). Hennessey Watson Associates was also was also a participating member in the firm of Consortium Designers Inc. during which time Hennessey was in charge of architectural design.

His work is representative of the architecture of the 1960s and 1970s, in particular, the design of schools and churches. He was active during the period of school consolidation in Prince Edward Island and designed many of the new schools as well as several buildings for St. Dunstan's University and its successor, the University of Prince Edward Island including the Duffy Science Building. Several of his projects involved senior citizen's housing, tourist facilities and industrial buildings. A few were for residences. Religious buildings of note include the Monastery of the Precious Blood and Holy Redeemer Catholic Church in Charlottetown. A major project was the new Public Administration Building for the Government of Prince Edward Island.

A. J. Hennessey retired in 1985.