Frank Hennessey was born in Ottawa, Ontario, in 1894. Hennessey did nature study drawings which were reproduced in The Ottawa Journal when he was only six years old. He attended St. Patrick's School , Ottawa Collegiate.
At the age of 14, accompanied Captain J.E. Bernier on an exploration trip in the Arctic.
From 1915 to 1917 he was an inspector of explosives with the Imperial Munitions Board. In 1921 Hennessey joined the entomology division of the Department of Agriculture in the Science Service section where he continued to work until his death.
Of his many trips to the wilds Hennessey found the Algonquin country an ideal sketching ground.
In 1934 Hennessey was elected an Associate of The Royal Canadian Academy.
In 1938 he was elected Fellow of the Royal Society of Arts London, England.
By 1940 Hennessey was exhibiting at the Stevens Art Gallery in Montréal where his work was appreciated by the Montréal Gazette. He exhibited at Wilson & Son annually.
Hennessey's work can be found at The National Gallery of Canada; The Sheffield Art Gallery (England); The Hall of Nations (Asbury Park, N.J., U.S.A.); The Art Gallery of Ontario (Toronto); The Art Association of Montreal; The Museum of the Province of Quebec; Lady Tweeds - muir (England); The Late Rt. Hon. R.B. Bennett (Canada); The Bishop Suter Art Gallery (Nelson, New Zealand); W.R. Mitchell (Montreal); Col. Willis-O'Connor (Ottawa); Mrs. R.A. Olmsted (Ottawa), Mr. & Mrs. Jules Loeb (Aylmer Road, Quebec), and in many other private collections in Canada, New Zealand, United States, Germany and Japan.