Leinster helped shape the foundations of modern science fiction. His landmark story "Sidewise in Time" (1934) was a pioneering work of alternate history, and the genre's prestigious Sidewise Award is named in its honor. "First Contact" (1945) established many conventions of human-alien encounter stories and won a Retro-Hugo Award. His remarkably prophetic tale "A Logic Named Joe" (1946) anticipated networked home computers decades before the internet became reality.
His major novels include "The Forgotten Planet" (1954), "The Pirates of Ersatz" (1959), "The Wailing Asteroid" (1960), and the popular Med Ship series featuring interstellar physician Calhoun. Beyond writing, Leinster was also an inventor who held several patents, including one for a front-projection television system.
In recognition of his lasting influence, the Science Fiction and Fantasy Writers of America named him their first Author Emeritus in 1995. Leinster remains a foundational figure of twentieth-century science fiction.