In her younger years, Mirajane's parents died and so she joined Fairy Tail along with her two younger siblings, Elfman Strauss and Lisanna Strauss. She was around 13 years old at that time. When she was younger, she was always in brawls with her rival of the time, Erza Scarlet. She even intimidated many of the older members, due to her punkish attitude and bullying tendencies. Despite teasing Natsu endlessly, she always had a soft spot for him, finding him to be cute when he cries. As she grew older, her abilities increased tremendously, and in the year X781, at the age of 16, she became an S-Class Mage, being the second youngest Mage in Fairy Tail to do so; only Erza was younger when she achieved the same honor.

One year after the promotion, during a mission with her younger siblings, Elfman and Lisanna, Lisanna was supposedly killed by Elfman, due to him losing control when he tried to take over "The Beast". The shock of Lisanna's loss caused Mirajane's personality to shift dramatically; she abandoned her bullying tendencies and became the sweet and motherly figure beloved by the guild today. She also lost her will to fight and the vast majority of her Magic Power.