Mary Batson Bromfield is the sister of William "Billy" Batson — the original Captain Marvel.

Billy Batson was hosting a a teenage quiz show named The Mental Marvel Quiz at Station WHIZ when he unknowingly first met his long lost sister, Mary. The quiz show contestants included the kid with the highest intelligence in the state, Percy Pill, Freddy Freeman who was representing the poor children and a certain Mary Bromfield who was the girls representative. During the quiz, Billy noticed that Mary was wearing an odd half-broken locket.

During a break in the quiz show, Billy was handed a urgent request to see a dying woman. With the quiz show still in a commercial break, Billy changed to Captain Marvel to quickly visit the woman. Billy greeted the bed-ridden woman, Sarah Primm. The deathly-ill Primm revealed that he had a long-lost twin sister and also told him that she was his and Mary's nurse when their parents were killed. Primm took both of them into her care but eventually applied to have them sent to an orphanage. Primm was also the nurse of a wealthy patient whose baby had died in child birth. Primm substituted Mary for the dead child so that Mary would have a good home even if Billy couldn't. She also told Billy that she had given Mary one half of a broken locket and then gave Billy the other half so that he would have a way of locating Mary. And then, suddenly, Primm died.

Billy, as Captain Marvel, raced back to Station WHIZ to continue the quiz show. After the show was over, Billy wished that Mary would turn out to be his sister and then remembered her broken locket. Determined to confront her, Billy and Freddy followed he car as Captain Marvel and Captain Marvel, Jr. only to discover Mary was being kidnapped. Inside the hideout Cap and Junior made short work of the kidnappers before revealing that Mary and Billy were twins. Mary was even more surprised to learn that Captain Marvel and Billy were one and the same as he and Junior transformed back into their teenaged selves.

The revived kidnappers took that opportunity to gag both Billy and Freddy, preventing them from changing into their super-powered selves. Mary, now fearful that the boys couldn't say their magic words accidentally said the word, "Shazam," aloud and was instantly transformed into a super-powered kid herself. Easily moping the floor with the hoodlums she ungagged Billy and Freddy. The three of them flew to the abandoned subway tunnel to visit old Shazam! The ancient wizard revealed that the siblings would one day be reunited and welcomed Mary into the Marvel Family. Shazam also revealed that Mary's source of power came from a different collection of gods: Selena, Hippolyta, Ariadne, Zephyrus, Aurora and Minerva!

After a 20 year hiatus, it was revealed that in his attempt to place Captain Marvel (and the rest of the Marvel Family) in suspended animation, Sivana also caught himself, the rest of the Sivana Family as well most of Captain Marvel's friends and family in the same trap.

Earth-S

Sivana used a compound of his own invention, called Suspendium, to pull off his nefarious feat. He transported the Cap and the other folks to space where he placed them in the Suspendium globe. Sivana's son, Sivana Jr., accidently caused Sivana's space ship to crash into the globe trapping themselves as well.

After 20 years, the Suspendium globe moved close enough to the sun and started to melt. Captain Marvel was the first to awaken and he freed the rest of Sivana's prisoners including Mary. In the end, Cap, Mary, and Junior captured the Sivana's and sent them back to jail.

It was during this time that it was revealed that the Marvels lived on Earth-S, a parallel world existing in a different vibratory space from the DC Universes of Earth-1 and Earth-2. Earth-1 and Earth-2 were homes to the silver and golden age DC heroes including Superman, Batman, Wonder Woman, and the rest. Mary had many solo and team adventures during this time.

Then a cataclysmic, universal catastrophe called the Crisis on Infinite Earths occurred. Mary joined the heroes from many earths to battle the evil Anti-Monitor - a being hellbent on destroying all the parallel universes and restarting the universe from scratch. Mary and the other heroes defeated the Anti-Monitor but not before the remaining earths were merged together. For the first time, Mary and her family lived on the same earth as Superman and Batman.

Post-Crisis

Mary's origin was slightly altered after the Crisis. In the new history, Billy discovers Mary before they both decide to share their powers with Freddy. Sarah Primm is still alive and Mary's nanny when Mary is kidnapped. And it is Mr. Tawky Tawny, a spirit inhabiting the shell of a stuffed animal, who reveals the magic word to Mary. Mary saves herself and Ms. Primm before Primm is shot down by the kidnappers. It was later revealed that Sarah Primm and Theo Adam (Black Adam) are sister and brother.

Mary met up with the Matrix Supergirl for road trip where they battled the evil ???. Mary got along with Supergirl but Supergirl was annoyed by Mary's optimistic attitude. Mary amicably parted ways with the Girl of Steel.

Mary Marvel's latest adventures have been with the Super Buddies, the cast off members of the reformed Justice League with Mary taking the place of her brother.

Mary proved to be the most powerful member of the Super Buddies, beating Captain Atom to a literal pulp during a mind-controlled battle staged by the villainous Roulette.

Unfortunately Mary's great wisdom, provided by Solomon, failed her during her adventures with the Super Buddies as it seemed her personality and reasoning were superceded by another god.

Mary was briefly seen at Sue Dibny's funeral during the Identity Crisis.