Thunder was created by Jerry Ordway and Mike Manley for the Legion of Superheroes. Her name is derived from Captain Marvel
co-creator C. C. Beck
CeCe Beck was born on the planet Binderaan in the 80th century to two loving parents who were unfortunately killed when she was a girl. During this same incident, CeCe became lost in a magical realm. The power of Shazam was bestowed upon CeCe not by the wizard Shazam himself, but his successor Captain Marvel, who, by the time of Beck's life (6000 years into the future), had become a bearded, immortal old man. As with her predecessor Captain Marvel, Jr., whenever Beck invoked the name "Captain Marvel," she was struck by magical lightning, aging her to an adult and bestowing upon her all the powers of the Marvel Family. She was returned to her world, where she was adopted.
She spent a mission in the past, specifically the 20th century. Thunder was then launched into the 30th century, when pro-science terrorists attacked and destroyed the Rock of Eternity, a major source of all magic. After subduing some of the terrorists with the help of members of the Legion of Super-Heroes she was offered membership, which she accepted. 'As a Legionnaire she continued her mission to collect all the scattered pieces of the Rock of Eternity. This caused her to often take long or detached assignments away from the team. Eventually, CeCe was able to gather many pieces and thanks to a mystical threat and his defeat, the Rock of Eternity was restored. It was during this struggle that she discovered that speaking the name 'Captain Marvel' as Thunder in the 30th century would actually spring her back to the 80th century as CeCe. This meant that she could reconnect with her foster family and home time while still serve as a Legionnaire in the past.
Soon after the Rock was restored, the alien Blight invaded Earth. Thunder was one of many Legionnaires "blighted" and turned into a mind-controlled slave. She was freed with her team but then Legion came under intense pressure and negative sentiment from the rest of the galaxy. CeCe took this opportunity to resign and return to her own time, promising the Legionnaires she would return one day. This promise wasn't fulfilled before Earth-247, the reality that housed the 30th century she visited, was erased.