Leinster was born in Norfolk, Virginia, the son of George B. Jenkins and Mary L. Jenkins. His father was an accountant. Although both parents were born in Virginia, the family lived in Manhattan in 1910, according to the 1910 Federal Census. He began his career as a freelance writer before World War I; he was two months short of his 20th birthday when his first story, "The Foreigner," appeared in the May 1916 issue of H. L. Mencken's literary magazine The Smart Set. Over the next three years, Leinster published ten more stories in the magazine. During World War I, Leinster served with the Committee of Public Information and the United States Army (1917-1918). During and after the war, he began appearing in pulp magazines like Argosy, Snappy Stories, and Breezy Stories. He continued to appear regularly in Argosy into the 1950s. When the pulp magazines began to diversify into particular genres in the 1920s, Leinster followed suit, selling jungle stories to Danger Trails, westerns to West and Cowboy Stories, detective stories to Black Mask and Mystery Stories, horror stories to Weird Tales, and even romance stories to Love Story Magazine under the pen name Louisa Carter Lee. Leinster's first science fiction story, "The Runaway Skyscraper," appeared in the February 22, 1919 issue of Argosy, and was reprinted in the June 1926 issue of Hugo Gernsback's first science fiction magazine, Amazing Stories. In the 1930s, he published several science fiction stories and serials in Amazing and Astounding Stories (the first issue of Astounding included his story "Tanks"). He continued to appear frequently in other genre pulps such as Detective Fiction Weekly and Smashing Western, as well as Collier's Weekly beginning in 1936 and Esquire starting in 1939. Leinster was an early writer of parallel universe stories. Four years before Jack Williamson's The Legion of Time came out, Leinster published his "Sidewise in Time" in the June 1934 issue of Astounding. Leinster's vision of extraordinary oscillations in time ('sidewise in time') had a long-term impact on other authors, for example Isaac Asimov's "Living Space," "The Red Queen's Race," and The End of Eternity.