Snape was raised in a poor area of the Muggle town of Cokeworth by his alcoholic Muggle father and depressed Witch mother. He attended Hogwarts from 1971 until 1978 as part of Slytherin house, where he gained an affinity for the Dark Arts and spell-crafting. Severus had been best friends with Lily Evans, a Muggle-borne, falling deeply in love with her, but pressure upon them because of them being in rival houses and the emotional strain of being victimized daily by a group of Gryffindors called the Marauders caused him to lash out at Lily, and their friendship self-destructed. Failing to win back the friendship of his true love, Snape had nowhere else to turn to and became closer to the blood supremacist students of Slytherin, becoming a Death Eater straight out of graduation at the age of 18.
After overhearing part of a prophecy concerning a certain child destined to bring about the end of the Dark Lord Voldemort, he relayed the information back to his master, horrified to discover that he unknowingly put Lily Evans and her then-unborn child in danger. Desperate to save her, Severus went to Albus Dumbledore and agreed to become a spy for the Order of the Phoenix in exchange for Lily's safety. For the remainder of the First Wizarding War, Snape kept his true loyalties hidden from Voldemort thanks to his incredible skill in Occlumeny. After the betrayal of the Potters and first defeat of Lord Voldemort, Snape agreed to look after Harry Potter, the true boy-who-lived, and placed him in the care of the Krum family.
Severus then met with fellow Death Eater defector Igor Karkaroff and took a position at Durmstrang Institute as its Understanding the Dark Arts Professor while Karkaroff became the school's Headmaster after Vitally Ivanov's sudden retirement. During this time, he married Mariya Kozlov, who unfortunately passed away in 1989. He kept a watchful eye on Harry Potter, now named Fedor Krum, during his time at Durmstrang. In 1994, he was part of the Durmstrang delegation to Hogwarts for the 1994 Triwizard Tournament. There, he did everything to help Fedor—who's name was somehow entered into the Goblet of Fire—overcome the tasks of the Tournament.
After the end of the tournament and the return of Lord Voldemort, Snape took over as Headmaster of Durmstrang after Igor Karkaroff went into hiding. He took it upon himself to hide Lily Evans in the castle and put increased importance into the self-defence courses of Durmstrang's curriculum. He confronted Lord Voldemort during his attempted takeover of Durmstrang in 1996 and led Durmstrang students in the defence of Hogwarts in 1997.