Sarah was born in Colchester, England. After receiving an art history degree and a doctoral thesis from the University of Cambridge, Gardner moved to Chicago to pursue a PhD. in archaeology at the University of Chicago in the 1990s.
While at the university, she worked in researching ancient Egypt alongside her colleagues Dr. David Jordan, Dr. Steven Rayner and Dr. Daniel Jackson. She began a romantic relationship with Daniel Jackson, but broke off with him after he forgot their anniversary. (SG1: "The Curse", "Chimera") (RPG: "Living Gods: Stargate System Lords")
Gardner, along with Jordan and Reyner, began to study the items of the Stewart expedition which were salvaged from the seas decades after they were sunk in 1931. She opened one of the items, the Osiris jar, which contained the sedated symbiote of the Goa'uld Osiris. When she opened the jar, she awoke the symbiote and he took her as his host. Osiris/Gardner managed to escape Earth aboard his ship. (SG1: "The Curse")
In 2003, Osiris/Gardner came back to Earth upon hearing that Daniel Jackson had returned from his ascended state. He had previously discovered a tablet which he thought would lead him to the Lost City of the Ancients and, as Daniel had been ascended, he decided to use him to translate it. However, SG-1 eventually caught on to the plan, and Osiris was captured. The Tok'ra subsequently extracted Osiris from Gardner at Daniel Jackson's request (SG1: "Chimera")
Most likely, Gardner was debriefed by the SGC and then returned to civilian life, as she does not appear to have joined the Stargate program.