In Wanda's past, she was born to a mobster fairy named Big Daddy and an unknown mother, and she became the hard-working daughter in the family, as opposed to her "identical yet somehow hotter" younger sister, Blonda, who went on to star in the soap opera All My Biceps. In high school, Wanda began dating an exchange student named Juandissimo Magnifico, but later broke up with him, probably due to his vanity.
Eventually, Cosmo fell in love with her, eventually proposing marriage to her with a chewed-up pen cap. But Cosmo's mother, Mama Cosma, really disliked Wanda, as she did not like the idea of her son settling down with anyone. In order to elope with Wanda, Cosmo ran away from home, telling his mother that he was going out to buy milk. By the events of the episode "Apartnership!", Cosmo and Wanda have been married for 9,895 years. A lot of things may have happened to them during that period, but most of them are unknown. What we do know is that they served as fairy godparents for Denzel Crocker in the early 1970's. Unfortunately, an incident involving their selves from thirty years later caused poor Denzel's secret to be revealed, and they were taken away from him. Their next assignment was evidently young Bill Gates.
Their time to care for Timmy Turner came about a year after he met Vicky and she began tormenting him. Timmy wasn't so positive about meeting Cosmo and Wanda at first, but after his first day with them, Timmy grew to love them like they were his best friends. Cosmo and Wanda usually transform into various animals and objects to avoid being seen by other people. Their most common disguise is as Timmy's pet goldfish. As Timmy's fairy godparents, they even gave birth to their own son, Poof, as a result of one of Timmy's wishes.
Besides the now-maniacal Crocker, the only other humans to know of Cosmo and Wanda's existence include Vicky's cute little sister Tootie (in the episode "Birthday Wish!" and the live-action movies), Remy Buxaplenty (Timmy's rival and Juandissimo's godchild), and fellow Nicktoon star Jimmy Neutron who, mistakenly believing that Timmy is also a genius, thinks Cosmo and Wanda are advanced holograms.
In the future depicted in the live-action telefilm, A Fairly Odd Movie: Grow Up, Timmy Turner!, Cosmo and Wanda manage to stay with Timmy thanks to his continuing to act like a child in his early adulthood. But when a now-beautiful Tootie comes back into his life, Timmy is driven to choose between remaining a mental child and keeping Cosmo and Wanda, or growing up and losing them so he and Tootie can be together. They and Poof are then taken captive by the evil oil tycoon Hugh J. Magnate Jr., but Timmy ultimately rescues them by confessing his love for Tootie, thus losing them due to officially growing up, but also freeing them from Magnate. As a result of Timmy saving them, a change is made to "Da Rules" in which Timmy is freely allowed to keep them as an adult, under the provision that he make unselfish wishes from now on, and Tootie is also allowed to know of their existence. Cosmo, Wanda, and Poof then go on to help Timmy and Tootie in their new family business, "Wishful Thinking".
The epilogue for the movie Channel Chasers shows that Cosmo and Wanda will apparently be fairy godparents to Timmy and Tootie's own children, Tommy and Tammy Turner, later in the future. However, the timeline made by Grow Up, Timmy Turner! may turn this into becoming the entire household's godparents.