She was raised in the Almoravid Palace in Marrakesh as the daughter of Umar ibn Yintan. Her future role was not strange, as women in Almoravid Morocco had a higher status than what was otherwise normal in Muslim states. Women at court were known to wield great influence in state affairs in the tradition and example of Zaynab an-Nafzawiyyah, co-founder of the dynasty: women in Morocco did not wear veils, and the education of women was accepted and normal, with notable women such as Hafsa Bint al-Hajj al-Rukuniyya holding courses for the women of the palace, and at least two women known to have been doctors.
In March 1147, the forces of Abd-al Mumin, successor of Ibn Tumart and founder of the Almohad Caliphate, reached the capital of Marrakech during his jihad against the Almoravid dynasty, broke into the town and battled over the control of the qasba (fortress) of the Almoravid Palace for days. Fannu famously dressed herself in male clothing and participated in the defense of the fortress, and according to tradition, the Almohads did not manage to conquer the fortress before "a young Almoravid woman, dressed as a man", was killed. The Almohads were reportedly astonished by Fannu's conduct and bravery during the battle, and surprised that they had not realized that she was a woman before she died.
In Morocco, she was depicted as a figure of Berber resistance.