Severus Snape becomes headmaster of Hogwarts in J.K. Rowling’s Harry Potter and the Deathly Hallows.
After Albus Dumbledore’s death, the Ministry of Magic installs Snape as headmaster during Lord Voldemort’s control of the wizarding government. Snape’s appointment is presented as part of the new regime’s takeover of Hogwarts, with Death Eaters Alecto and Amycus Carrow enforcing its ideology at the school.
Snape’s position is more complicated than it first appears. He is secretly working against Voldemort and trying to protect students as far as possible while maintaining his cover. Harry learns the full story only after Snape’s death, when memories reveal his long loyalty to Dumbledore.
Minerva McGonagall later leads Hogwarts during the final battle and becomes headmistress after Voldemort’s defeat. That later role is why readers sometimes give McGonagall instead.