Born in the suburbs of Liverpool on May 21, 1885, Arthur was the son of a Cunard Line steward; as an only child, he grew up surrounded by Cunard uniforms and receipts. In 1899, at the age of 14, he briefly worked for the Union-Castle Line as an apprentice steward qualifying professionally shortly thereafter, and in 1905, he left Union to join the White Star Line, taking a position as a steward aboard the Oceanic, just like his father. That same year, his father, Walter Whitmore, passed away from tuberculosis. In 1910, Arthur left his steward role to become a cabin steward on the Megantic and later on the Baltic. In September 1911, following the arrival of the Olympic, he was transferred to the new ship; he was present during the Olympic's collision with the Hawke, an event that inspired him to begin writing about his adventures at sea. In March 1912, there was talk of transferring him to the Titanic, as was the case for most of his colleagues from the Olympic, but he remained on the Olympic. He was aboard the Olympic when the Titanic sank.

At the end of that year, Arthur lost his mother and spent over five months away from the sea, returning in June 1913 aboard the Adriatic. The following year, with the outbreak of the Great War, Whitmore was slated to serve as a steward on the Oceanic again; however, he remained on the Adriatic before transferring to the Cedric in September 1914, where he stayed until the ship's final commercial voyage in October of that year. It was during this period that Arthur’s life was nearly derailed by rumors of a relationship with a passenger during one of the Cedric’s last voyages. Although Arthur denied the allegations, he was suspended by the company for more than three months. He returned to duty the following year his personality now changed and more stern, serving as an assistant steward on a voyage aboard the Zealandic until November 1915, when he was transferred to another vessel as a steward: the hospital ship Britannic, the newest member of the Olympic class. Arthur remained with the ship until its sixth voyage, which began on November 12, 1916; he had intended to retire from the profession after that trip.