Grese was convicted for crimes involving the ill-treatment and murder of prisoners committed at Auschwitz and Bergen-Belsen concentration camps, and sentenced to death at the Belsen trial. Executed at 22 years of age, Grese was the youngest woman to die judicially under British law in the 20th century. Auschwitz inmates nicknamed her the "Hyena of Auschwitz" (German: die Hyäne von Auschwitz).

Irma Ilse Ida Grese was born to Berta Grese and Alfred Grese, a dairy worker, on 7 October 1923. Irma was the third of five children (three girls and two boys). In 1936, her mother committed suicide by drinking hydrochloric acid after discovering that Alfred had had an affair with a local pub owner's daughter.

Historian Peter Vronsky speculated that Alfred Grese joined the Nazi Party in 1937, and remarried in 1939.

She left school in 1938 at age 14, probably due to a combination of a poor scholastic aptitude, bullying by classmates, and a fanatical preoccupation with the League of German Girls (Bund Deutscher Mädel), the Nazi female youth organization, of which her father disapproved. Among other Casual jobs, she worked as an assistant nurse in the sanatorium of the SS for two years and unsuccessfully tried to find an apprenticeship as a nurse.

The night before her execution, Grese sang Nazi hymns until the early hours of the morning with Johanna Bormann. On Thursday, 13 December 1945, in Hamelin Jail, Grese was led to the gallows. The women were executed singly by long-drop hanging and then the men in pairs. Regimental Sgt. Maj. O'Neill assisted the British executioner, Albert Pierrepoint:

In this function she participated in prisoner selections for the gas chambers. Soon after gaining power, she gained a reputation as a sadist. She was considered to be a nymphomaniac who slept with SS guards as well as prisoners—the latter, forcibly. She bribed young Jewish girls to stand watch for her as she raped other inmates, and, according to inmate and Doctor Gisella Perl, Grese “relished whipping well-developed young women on the breasts… Grese would eventually become sexually aroused just watching the women suffering."

Grese inspired virulent hatred in prisoner Olga Lengyel, who wrote in her memoir Five Chimneys that selections in the women's camp were made by SS Aufseherin Elisabeth Hasse and Irma Grese. The latter was visibly pleased by the terror her presence inspired in the women at roll call. Grese had a penchant for selecting not only the sick and the weak but any woman who had retained vestiges of her former beauty. Lengyel said Grese had several lovers among the SS in the camp, including Josef Mengele. After Grese forced the inmate surgeon at the infirmary into performing her illegal abortion, she disclosed that she planned a career in film acting after the war. Lengyel felt that Grese’s meticulous grooming, custom-fitted clothes and overuse of perfume were part of a deliberate act of sadism directed toward the ragged women prisoners.