Amir was born in Herzliya to an Israeli Orthodox Yemenite Jewish family, one of eight children. His father Shlomo was a sofer who held a post supervising the kosher slaughtering of chickens and taught Shabbat lessons at a local synagogue. His mother Geula was a kindergarten teacher, and ran a nursery school in the family home's backyard. Amir attended an Independent Education System school in Herzliya, and a high school yeshiva in Tel Aviv. He did his military service in the Israel Defense Forces as a "Hesder" student, combining army service in a religious platoon of the Golani Brigade with religious study at "Yeshivat Kerem B'Yavneh". Despite being in a religious unit, even his comrades considered him a religious fanatic.