Samuel Spade was a British emigrant who was brought to the United States of America by his parents Joshua Spade and Mary Brandon while still a boy. They settled in Indiana, and Samuel joined the police on reaching adulthood. He married a Faith Shawnessy, and had a son named John Spade - father of Dashiell Hammett's Sam Spade - who joined the Pinkertons. The family are discussed in Doc Savage: His Apocalyptic Life by Philip José Farmer.
In his book Crossovers, Win Scott Eckert notes that in the novel Slaughter Road by George C. Gilman (1975), a Samuel Spade has been working as a partner in San Francisco private enquiry agency in the years leading up till 1875, and had worked on what sounds like an early incident in the family's involvement with the Maltese Falcon. Mr. Eckert suggests that the two Samuel Spades are one and the same person, and that he joined the police after the events of Slaughter Road.