Sloan was born in Pretoria, South Africa, at least per his Starfleet Intelligence records. On his application, he had written that his ultimate ambition was to be Director of Starfleet Intelligence -- a notion that then-Director, Admiral Nyota Uhura, found amusing. In 2360, Sloan served as a courier. (The Lost Era novel: Catalyst of Sorrows)
In late-2374, Sloan attempted to recruit Julian Bashir into Section 31 by testing Bashir in a simulation where Bashir was accused of being a spy for the Dominion. In the simulation Sloan took the role of the deputy director of Department of Internal Affairs. (DS9: "Inquisition")
Sloan was known to use aliases. One of these was Wendell Greer, a mid-level bureaucrat in the Federation Department of Cartography. Sloan used this alias in 2375 while attending the Federation-Romulan Conference on Romulus. (DS9: "Inter Arma Enim Silent Leges")
After Section 31 heard that Bashir had apparently cured the morphogenic virus that was killing the Founders, Sloan traveled to Deep Space 9 to destroy the cure. It proved to be a trap and Sloan was captured. To prevent Bashir from getting the information through the use of Romulan mind probes Sloan triggered an implant in his brain to commit suicide. Sloan's actions were fruitless as Bashir was able to keep Sloan alive long enough to enter his mind and retrieve the information before Sloan could die. (DS9: "Extreme Measures")
In 2378, Sloan was still alive despite appearing to have taken his own life three years earlier continuing to conduct covert operations for 31. He posed as the deceased Verad Kalon in order to infiltrate the Neo-Purists to learn the rebel Trill groups connection to renegade Klingons known as the Ku-Vok-leth. As Kalon, Sloan allowed Benjamin Sisko to infiltrate the Neo-Purists, introducing Sisko as Benjamin Russell to his cohorts.
After the Neo-Purists' attempt to sabotage Deep Space 9 was revealed as a ruse to smuggle explosives aboard the IKS Sword of Kahless in hopes of assassinating Chancellor Martok, Sloan/Kalon was arrested and interrogated by station security. Sisko would later be coerced into breaking Kalon out of prison when an Orion Syndicate operative threatened Sisko's family. Once Sisko thwarted Runold's attempt to kill Sloan/Kalon, Sloan was suddenly beamed off the station by Section 31 Director L'Haan. Once Sloan's true appearance was restored, he explained to L'Haan that this operation was to pique the interests of Julian Bashir and his colleagues to demonstrate that Section 31 was seeking to assure the Federation's survival amid a larger crisis. (Star Trek: War Aftermath: Episode 2: Omega)
Sloan posed as a computer technician at the Daystrom Institute of Psychiatric Health in 2374. His assignment there was erase all data a group of genetically-enhanced residents had compiled on the Dominion War and replace it data on theories about the possible end of the universe. At that time, Sloan had a chance encounter with Sarina Douglas, who was in a semi-cataleptic state at the time. He expressed fascination with Douglas's condition, even informing her of Julian Bashir's eventual efforts to free her from her "prison".
Sloan served as both first officer and tactical officer of the USS Damocles in the guise of Jack McGarrett, a legitimate member of Starfleet Intelligence, in 2375. During a number of engagements with the Dominion in the Romulan Neutral Zone, Sloan stumbled across evidence that the Tal Shiar was conducting research on thalaron radiation on the planet Goloroth. He then struck up a deal with General Valnor of the Tal Shiar to keep quiet about the thalaron generator in return for assistance in 31's thalaron research. Sloan had also threatened to expose the Tal Shiar's role in the death of Vice admiral Fujisaki, the deputy chief of Starfleet Intelligence, if Valnor or Tal Shiar Chairman Koval were to double-cross him. (Star Trek: War Aftermath Episode 2, "Omega")