The son of John Thomas and Annie Onions, Forrest was born on 20 June 1896 in San Francisco, California, in the United States. His records in America have his surname as “Wenyon”, whereas the UK records list his surname as “Onions”. By 1901, the family had moved to Bleak House, Mount Pleasant, Bilston, where Forrest lived with his parents and siblings Muriel, Kathleen, Florence, Arnold and Doris. By 1911, they were living at 88 Wellington Road, Westbourne, Bilston, and Forrest had an additional two sisters, Eileen and Mildred. Forrest had become a draughtsman for a motor works in Wolverhampton.
Forrest enlisted in the Royal Flying Corps, later the Royal Air Force, under service number 4262. He survived the war, and returned to the United States. On 14 October 1943, he married Margaret C. Hubbard Beach in Lewes, Sussex, Delaware. He died in Delaware on 28 October 1974.