Remus was born on 10 March, 1960 to Lyall and Hope Lupin. His father worked at the Ministry of Magic and encountered the werewolf Fenrir Greyback, who was on trial for killing two children. Lyall was the only one at court to realise that Greyback was a werewolf, as Greyback pretended to be a Muggle tramp. Outraged when Greyback was released, he voiced the opinion that Greyback deserved nothing but death. This opinion cost the Lupin family dearly, as Greyback decided to revenge himself upon Lyall by targeting his son. As the nearly five-year-old Remus slept peacefully in his bed, Greyback forced his way in through the window. Though Lyall was able to get there in time to drive Greyback off with powerful spells, he was unable to prevent him from completing his evil goal. Remus became infected with lycanthropy and he became a werewolf himself.

His parents took him to various healers and tried their best to make him a normal boy, but there was no cure for his condition. But Headmaster Albus Dumbledore made special accommodations allowing Remus to attend Hogwarts School of Witchcraft and Wizardry.