Born Mary Creamer, the future Gimmick Girl was the daughter of "Fly-foot" Creamer, an acrobat who used "human fly" methods to commit robberies. When he was finally apprehended, he had his daughter placed in an orphanage, stipulating that she must never know her father was a criminal. Mary remained in the orphange for 10years.
And there she might have grown up had it not been for a mishap that occured while the Star Spangled Kid and Stripesy were battling a gang of crooks. A bullet creased the kid's head, and while the injury was not serious, it meant that in his other identity of Sylvester Pemberton Jr, he had to wear a bandage, and since his parents were unaware of his dual ID, he had a hard time explaining.The elder Pemberton insisted on his son seeing a psychiatrist, who, noting that young Syl had no close friends or siblings, suggested the Pembertons adopt someone to be his companion. They chose Merry.
With his new sister hangins around, Syl and the family chauffer, Pat (Stripesy)Duggan, found it difficult to operate as a crime fighting team. Meanwhile, learning of the adoption, two hoods called Klinker and Buggsy went to Creamer, now out of prison and going straight as a circus performer, threatening to reveal to the Pembertons who Merry's real father was unless he helped them to pull a job. "Fly-foot" agreed, but he double crossed the hoods. They shot him, but just then Star Spangled Kid and Stripesy showed up. Fending off a live wire with a monogrammed handkerchief Merry had given him, the Kid got to Creamer to hear his last words, "They wanted to hunt my girl - she's with a nice family...Pembertons! I'm an ex-con - don't tell her, help her." The Kid and Stripesy agreed not to tell Merry about her dad, and she never did learn. The killers died in a shoot - out with the police.
But she did hear of the hankie the Kid had used, and so learned his secret ID. She kept the secret and even made a costume for herself, in red, white and blue, like those of the KId and Stripesy. But while the Kid's outfit was mainly blue, and Stripesy's mainly red, Merry's was largely white.Merry secretly aided the Kid in capturing Presto the Magician, and it seems his gimmicks inspired her, for afterwards she was never without surprising gimmicks she created to use against villains. When Pat broke his leg battling a crook called the Rope, Merry first used her gimmicks, capturing the Rope when he had kayoyed the Kid.
Merry continued to fight crime for a while as the Girl of a thousand gimmicks, and it may have been during one of her exploits that she met Henry King (Brainwave). He was much older than her, but they fell in love and were married - and they had a son - Henry Jr who became the hero Brainwave Jr. in Infinity Inc. When her husband was arrested for his continual crimes, Merry had to raise Henry Jr. alone. When Henry was in his early teens, Merry succumbed to the strain she had been under for years and died at a relatively young age.
However, Merry's apeared years later as a member of Old Justice. The only explanation she gave for being alive was that rumors of death are often untrue.
Though she attempted to blackmail the Red Tornado into helping her group shut down Young Justice, she later proved that she only wanted to help the rookie heroes.