Gregory was born in Devonshire, England, in 1829. He became a renowned physician as well as a researcher of the Abnormal world. By the late 19th century he was considered one of the top scientists of his day. He married a woman named Patricia Heathering, who gave birth to a daughter, Helen, in 1850. Helen followed in his footsteps as a doctor, and after she was denied any opportunities to actually practice, was introduced to the world of the abnormals by him.
He later disappeared during an expedition to Mecca; Helen believed him to be dead, but he had in fact found a way into Hollow Earth and lived with the people of the underground city of Praxis for many years. Eventually, he tried to return to the surface in order to make contact with Helen, but was kidnapped and enslaved by the Cabal.
In 2009, he reappeared under the name Charles L. Denton and without any memory of his former life. After Helen learns that he was now himself an Abnormal and that a parasite called "scarab" is controlling him, and removes it, he regains his original memories, and decides to set out on his own, not wanting to endanger his daughter and the Sanctuary Network.
In 2010, he was seen in a vision by Will Zimmerman after communicating with Kali. He gave Will two numbers and a symbol which led Helen Magnus to unlock a holographic map of a city, therefore starting a new mission for the Sanctuary team.
Magnus and her team later encountered Gregory alive and well in Hollow Earth, where he had slowly earned the trust of the people of Praxis, including their leader, Ranna Seneschal. His contacting Helen and giving her the clues to find Hollow Earth was a transgression that was forgiven only after the team helped save the life of the superspecies abnormal Kanaan and defeat Adam Worth. Gregory decided to stay in Praxis for the time being in order to learn more about the secrets of Hollow Earth.
Gregory Magnus was presumably killed when Praxis was destroyed by Adam Worth in an attempt to power up the Colorem device, a machine he was building to travel back in time to save his daughter's life. However, Helen Magnus and John Druitt only recovered his broken walking stick, not his body.
There is some confusion as to how Gregory Magnus has lived as long as he has. He does not have his daughter's gift of longevity; yet he continues to outlive her supernaturally advanced age. Sam Egan, one of the only two writers of the first season, addressed this in a commentary on the episode "Warriors," in which Gregory Magnus was reintroduced into the story. Egan stated that he purposefully neglected to provide an explanation for Gregory's apparent lack of aging, but wanted to hint to the audience that he was in fact cryogenically frozen by the Cabal. However, this was later retconned when Gregory revealed that he had spent 65 years in the city of Praxis in Hollow Earth and achieved his longevity thanks to the advanced medicine there.