On April 24, 1895, he set sail from Boston, Massachusetts. In his famous book, Sailing Alone Around the World, now considered a classic of travel literature, he described his departure in the following manner:
I had resolved on a voyage around the world, and as the wind on the morning of April 24, 1895 was fair, at noon I weighed anchor, set sail, and filled away from Boston, where the Spray had been moored snugly all winter. The twelve o'clock whistles were blowing just as the sloop shot ahead under full sail. A short board was made up the harbor on the port tack, then coming about she stood to seaward, with her boom well off to port, and swung past the ferries with lively heels. A photographer on the outer pier of East Boston got a picture of her as she swept by, her flag at the peak throwing her folds clear. A thrilling pulse beat high in me. My step was light on deck in the crisp air. I felt there could be no turning back, and that I was engaging in an adventure the meaning of which I thoroughly understood.
After an extended visit to his boyhood home at Brier Island and visiting old haunts on the coast of Nova Scotia, Slocum departed North America at Sambro Island Lighthouse near Halifax, Nova Scotia on July 3, 1895.
Slocum intended to sail eastward around the world, using the Suez Canal, but when he got near Gibraltar he realized that sailing through the southern Mediterranean would be too dangerous for a lone sailor because of the piracy that still went on there at that time. So he decided to sail westward, in the southern hemisphere. He headed to Brazil, and then the Straits of Magellan. At that point he was unable to start across the Pacific for forty days because of a storm. Eventually he made his way to Australia, sailed north along the east coast, crossed the Indian Ocean, rounded the Cape of Good Hope, and then headed back to North America.
Slocum navigated without a chronometer, instead relying on the traditional method of dead reckoning for longitude, which required only a cheap tin clock for approximate time, and noon-sun sights for latitude. On one long passage in the Pacific, Slocum also famously shot a lunar distance observation, decades after these observations had ceased to be commonly employed, which allowed him to check his longitude independently. However, Slocum's primary method for finding longitude was still dead reckoning; he recorded only one lunar observation during the entire circumnavigation.
Slocum normally sailed the Spray without touching the helm. Due to the length of the sail plan relative to the hull, and the long keel, the Spray was capable of self-steering (unlike faster modern craft), and he balanced it stably on any course relative to the wind by adjusting or reefing the sails and by lashing the helm fast. He sailed 2,000 miles (3,200 km) west across the Indian Ocean without once touching the helm.
More than three years later, on June 27, 1898, he returned to Newport, Rhode Island, having circumnavigated the world, a distance of more than 46,000 miles (74,000 km). Slocum's return went almost unnoticed. The Spanish–American War, which had begun two months earlier, dominated the headlines. After the end of major hostilities, many American newspapers published articles describing Slocum's amazing adventure.
Per i due anni successivi lavorò come maestro d'ascia, fino ad un giorno di mezzo inverno del 1892, quando iniziò la storia del leggendario Spray. Tornato a casa, completamente rovinato, ebbe una strana offerta da un vecchio comandante cacciatore di balene: gli regalava il suo sloop in disarmo. Il giorno dopo scoprì che si trattava di un relitto. Ricostruì da solo lo Spray, così si chiamava quel vecchio legno risalente al 1801. Scrisse nel suo libro: "Lo feci saldo, robusto e bello. Chiglia, braccioli e ordinate di dura quercia. Il fasciame era di pino della Georgia, spesso tre centimetri e mezzo. Le impavesate fatte con scalmotti di quercia bianca, i corsi di coperta di pino bianco".
La sera del 24 aprile 1895, Slocum salpò con lo Spray da Boston senza una meta precisa. Tre anni, due mesi e due giorni dopo, il 27 giugno del 1898, dopo una traversata di circa 46.000 miglia, alle 01:00 gettò l'ancora nel porto di Newport. Aveva circumnavigato il globo, da est verso ovest, passando per lo stretto di Magellano e la Terra del Fuoco.
Il suo terzo libro Solo, intorno al mondo ebbe un buon successo e permise a Slocum di acquistare la sua prima casa in terraferma. Come tutti i grandi miti della storia, la sua morte è avvolta nel mistero. Nel 1909, all'età di 65 anni, Joshua Slocum salpò per l'ultima volta con lo Spray verso le Indie Occidentali. Non arrivò mai. Né Joshua Slocum né lo Spray furono più ritrovati.