College student Mary Maxwell loved reading comic books and longed to be a super hero instead of spending her days assisting her scientist father, Phillip Maxwell. But she didn't know was that her father was working for a secret group called STEALTH on genetic modification (which was designed to help people survive time travel). One day Mary and her father were attacked at their own house and escaped successfully on a rocket-propelled car. Just as they were far enough away, her father admitted everything about working for STEALTH. When he discovered their true purpose was to commit crimes in the past and escape to another time they threatened to kill his daughter if he stopped working for them.
They took a boat to continue their escape from the STEALTH agents and soon were lost at sea in a green fog. Mary fell off the side of the boat feeling the green mist infusing her body. Her father pulled her out of the water, and soon she began suffering from constant fatigue. Her father noticed that hummingbirds have a rapid metabolism, and designed a serum based on hummingbird DNA to increase her own slowed metabolism. He only intended to give her a small dosage. Unfortunately just as her father was injecting her with the serum they shot him, and he pumped the full dose of serum, altering her DNA. Mary suddenly discovered that she had superhuman speed. She fought off the STEALTH agents but was completely exhausted when she finished using her new power. She began testing her abilities, seeing how far and how fast she could run before she was exhausted. She discovered that her speed was nearly limitless, even running so fast she ran past herself (traveling though time). Soon she designed her very own costume and became the Flash
The Flash tracked down STEALTH headquarters and battled its agents, electrocuting their superhuman strong warrior Zorgul just as her strength was leaving her after a heated battle. Flash then watched as STEALTH’s leader, Andrea Zakara, emerged from a time machine that her father designed and instantly aged to death, victim of the very process Andrea had hoped to prevent using Mary's father's research.
Later Mary would join the Justice League of America, when they were assembled by the machinations of an orphan boy called Adam Strange.
Mary has the ability to run at near-light speeds for a short period of time before she burns through her body's energy reserves. She can even use her speed to run forward or backwards in time