She and her older sister Alma were drawn to the study of the occult, and this interest led them in the spring of 1918 to pay a visit to Aleister Crowley, who was living at the time in the Greenwich Village neighborhood of Manhattan. Crowley and Hirsig felt an immediate and instinctive connection. Leah asked him to paint her as a "dead soul" and in fact Crowley painted several portraits of her.
In 1919, after seeking out Aleister Crowley due to her interest in the occult, she was consecrated as his Babalon or, "Scarlet Woman", taking the name Alostrael, "the womb (or grail) of God." Leah Hirsig wrote in her 1921 diary: "I dedicate myself wholly to The Great Work. I will work for wickedness, I will kill my heart, I will be shameless before all men, I will freely prostitute my body to all creatures."
Leah had previously been married to Edward Hammond, by whom she had a son, Hans Hammond (13 Nov 1917-Oct 1985).
She then helped found the Abbey of Thelema with Crowley in Cefalù, Italy.
At the Abbey, Hirsig was instrumental in guiding Crowley, the Prophet of the New Aeon, to a deeper understanding of the Law of Thelema.
Leah Hirsig died in 1975 in Meiringen, Switzerland.