Little is known about Sark's background. He spent most of his youth in England, although a voice analysis reveals he spent a lot of time in Galway, Ireland. He is the heir to a vast fortune from his Romanov-descended father, Andrian Lazarey. He routinely changes his alliances, and his true allegiance seems to be only to himself. He is notorious for his smarmy attitude and his ability to evade and escape capture. Sark is a connoisseur of fine wine and may be considered sexually kinky (having once expressed a strong interest in having sex with a woman while she wore a latex Sydney Bristow mask).
SD-6 learns of a meeting scheduled between the organization and K-Directorate to discuss the sharing of Rambaldi technology. Sydney and Dixon are tasked to eavesdrop on the meeting. Sark offers to transfer $100 million to K-Directorate's Cayman Islands account. In exchange, he demands the Rambaldi manuscript (which includes Page 47). The offer is rejected, and Sark instantly assassinates Ilyich Ivankov, leader of K-Directorate. The suddenly-promoted new leader of K-Directorate, Lavro Kessar, accepts the offer. Sydney, who is dangling outside the window spying on the meeting, is spotted, a firefight ensues and Sark slips away. He is next seen in Tunisia trying to retrieve the Rambaldi manuscript, but Sydney gets to it first.
Sark travels to Denpasar to buy a second ampule of Rambaldi liquid so he can expose a Rambaldi document (possibly the artifact retrieved from FTL). Unknown to him, the buy is a sting set up by Sydney and Michael Vaughn. The buy is disrupted by Dixon and other SD-6 agents. Sark flees with the ampule but Vaughn captures him and recovers it. When Vaughn abandons Sark to rescue Sydney, SD-6 recovers him.
While in SD-6 custody Sark agrees to lead SD-6 to Khasinau in his Paris nightclub to recover the Rambaldi document. Sydney and Dixon are tasked to retrieve the document and replace it with a forgery. Sydney is further tasked by Vaughn to switch out the document with a counterfeit, but is unable to because she spots Will Tippin, who's been brought to Paris to meet a mysterious contact to discuss "The Circumference," in the club. Dixon recovers the document and Sydney extracts Will and, with Jack's aid, returns him to Los Angeles.
Sark escapes from the club, but Sloane had previously laced Sark's wine with a radioactive isotope and is tracking his location by satellite. However, Sark becomes aware of this and undergoes a full blood transfusion in Geneva. While SD-6 is on a wild goose chase to Switzerland, Sark actually returns to Los Angeles and kidnaps Will Tippin from a CIA safehouse. To secure Tippin's release, Jack steals the ampule from the CIA and Sydney steals the document from SD-6. Jack and Sydney expose the page and realize that it's The Circumference, instructions on how to use the Mueller device. Jack travels to Taipei and makes the exchange, and Sark vanishes into the night.
Sark carries out missions for the Covenant, sometimes with the aid of Lauren Reed, a double agent positioned within the NSC and married to Vaughn. One mission he undertakes with Allison Doren. With the apparent destruction of Project Helix, Doren is trapped in Francie's form but Sark, who claims to love her, says it doesn't matter. Doren is killed by Tippin in the course of that mission.
Sark and Reed become lovers and eventually conspire to assassinate the leaders of the six Covenant cells in an attempt to take control of the organization. They manage the assassinations and are congratulated by McKenas Cole, who has become second-in-command of The Covenant. He appoints them co-leaders of the North American cell. They ally once again with Arvin Sloane to obtain the Rambaldi equation that Nadia Santos, "The Passenger," channels but are unsuccessful. Reed carries out a mission disguised as Sydney to obtain the equation from the CIA and bomb the CIA headquarters while Sark monitors her. She escapes but he is captured.
Sark later teams with Sloane and Kelly Peyton to obtain The Horizon, the final Rambaldi artifact Sloane needs to complete his endgame, from Irina in exchange for two missiles. After planting a bomb to destroy APO headquarters (killing Thomas Grace), Sark accompanies Sloane to Mongolia to excavate the tomb of Milo Rambaldi and becomes involved in a stand-off involving Vaughn, Jack Bristow and other of Sloane's operatives. In a gun battle in Rambaldi's tomb, Sark retrieves the Horizon and returns it to Irina in Hong Kong. On Irina's orders, Sark prepares to launch the missiles at Washington, D.C. and London but is captured and shot by Vaughn. He gives Vaughn the codes to abort the launch and Vaughn apparently allows the wounded Sark to depart.
Several years later, Marcus Dixon approaches a semi-retired Sydney and Vaughn and asks for their help with a field assignment. It's revealed that Sark has continued his unbroken track record of survival and is behind the incident Dixon is seeking help with. Sydney jokingly chides Vaughn for allowing Sark to get away years earlier.