Joshua Slocum was born on February 20, 1844, in Wilmot Township, Nova Scotia, Canada, the son of John (1811-1887) and Sarah Jane (Southern) (1813-1860) Slocumbe. Slocum's seafaring career began as he and a friend signed on at Halifax as ordinary seamen on a merchant ship bound for Dublin, Ireland. From Dublin, he crossed to Liverpool to become an ordinary seaman on the British merchant ship Tangier, bound for China. During two years as a seaman, he rounded Cape Horn twice, landed at Batavia (now Jakarta) in the Dutch East Indies, and visited the Maluku Islands, Manila, Hong Kong, Saigon, Singapore, and San Francisco. While at sea, he studied for the Board of Trade examination, and, at the age of eighteen, he received his certificate as a fully qualified Second Mate. Slocum quickly rose through the ranks to become a Chief Mate on British ships transporting coal and grain between the British Isles and San Francisco.

In 1865, he settled in San Francisco, became an American citizen, and, after a period of salmon fishing and fur trading in the Oregon Territory of the northwest, he returned to the sea to pilot a schooner in the coastal trade between San Francisco and Seattle. His first blue-water command, in 1869, was the barque Washington, which he took across the Pacific, from San Francisco to Australia, and home via Alaska. He sailed for thirteen years out of the port of San Francisco, transporting mixed cargo to China, Australia, the Spice Islands, and Japan. Between 1869 and 1889, he was the master of eight vessels, the first four of which (the Washington, the Constitution, the Benjamin Aymar and the Amethyst) he commanded in the employ of others. Later, there would be four others that he himself owned, in whole or in part.

Shortly before Christmas 1870, Slocum and the Washington put in at Sydney. There, in about a month's time, he met, courted, and married a young woman named Virginia Albertina Walker (1849-1884). Their marriage took place on January 31, 1871. Miss Walker, quite coincidentally, was an American whose New York family had migrated west to California at the time of the 1849 gold rush and eventually continued on, by ship, to settle in Australia. She sailed with Slocum, and, over the next thirteen years, the couple had seven children, all born at sea or foreign ports. Four children, sons Victor (1872-1949), Benjamin Aymar (1873-1965), and James A. Garfield (1881-1955), and daughter Jessie (1875-1960) survived to adulthood. In 1884, Virginia became ill aboard the Aquidneck in Buenos Aires and died.

In 1886, at age 42, Slocum married his 24-year-old cousin, Henrietta "Hettie" Elliott (1862-1952). The Slocum family, with the exception of Jessie and Benjamin Aymar, again took to the sea aboard the Aquidneck, bound for Montevideo, Uruguay. Slocum's second wife would find life at sea much less appealing than his first. A few days into Henrietta's first voyage, the Aquidneck sailed through a hurricane. By the end of this first year, the crew had contracted cholera, and they were quarantined for six months. Later, Slocum was forced to defend his ship from pirates, one of whom he shot and killed; he was tried and acquitted of murder. Next, the Aquidneck was infected with smallpox, leading to the death of three of the crew. Disinfecting of the ship was performed at considerable cost. Shortly afterward, near the end of 1887, the unlucky Aquidneck was wrecked in southern Brazil.

From the wreckage of Aquidneck, Slocum built a 35-foot sailing canoe, Liberdade, in which he and his family sailed to New York. For the next several years he was not too successful financially. In December 1893, he was hired to bring the Ericsson Destroyer to Brazil to participate in a revolution. Unfortunately the Brazilians sank Destroyer at Bahia (Salvador) and Slocum was never paid for his services.

In 1894 Slocum acquired a sloop at Fairhaven, Massachusetts, which he rebuilt into the Spray, in which he single-handedly circumnavigated the globe. Slocum began his voyage from Boston, on April 24, 1895, and sailed to Westport (Brier Island), Nova Scotia, where he had Spray re-caulked. His intent was to sail to Pernambuco (Recife) Brazil, then down the east coast of South America through the Straits of Magellan and into the Pacific. However, Slocum decided to sail to Fayal, then to Gibraltar, though the Suez Canal and follow the west-east route. Upon his arrival at Gibraltar he was advised that the Red Sea was littered with pirates and it would be unsafe to travel there in a small unarmed craft. Taking this advice he redirected his voyage, following his original course. During his travels Slocum spend extended periods in Australia and South Africa, completing his journey around the world at Newport, Rhode Island, on June 27, 1898. Upon his return he retired and spend the remainder of his life writing and speaking about his adventure. In 1899 he published his account of the epic voyage in Sailing Alone Around the World, first serialized in The Century Magazine and then in several book-length editions.

In 1901, Slocum's book revenues and income from public lectures provided him enough financial security to purchase a small farm in West Tisbury, on the island of Martha's Vineyard, in Massachusetts. After a year and a half, he found he could not adapt to a settled life and Slocum sailed the Spray from port to port in the northeastern US during the summer and the West Indies during the winter, lecturing and selling books wherever he could. Slocum spent little time with his wife on the Vineyard and preferred life aboard the Spray, usually wintering in the Caribbean.

By 1909, Slocum's funds were running low; book revenues had tailed off. He prepared to sell his farm on Martha's Vineyard and began to make plans for a new adventure in South America. He had hopes of another book deal. On November 14, 1909, Slocum set sail for the West Indies on one of his usual winter voyages. He had also expressed interest in starting his next adventure, exploring the Orinoco, Rio Negro and Amazon Rivers. Slocum was never heard from again. In July 1910, his wife informed the newspapers that she believed he was lost at sea. In 1924, Joshua Slocum was declared legally dead.