His hands-off approach to spy work places him on the periphery of the numerous cliques and rivalries that form within the Circus, allowing him to survive multiple bureaucratic shakeups that occur throughout the 1960s and 1970s; nevertheless, he retains a secret loyalty to George Smiley, who functions as his mentor.
Guillam survives the purge of personnel loyal to Smiley, but is demoted to the head of the Brixton "scalphunters", fieldmen used in operations that require physical action and/or violence.
One of Guillam's agents, Ricki Tarr, inadvertently learns that the failure of Operation Testify—and Smiley's subsequent termination from the Circus—was due to the presence of a mole in the highest echelons of British intelligence.
When contacted by Tarr and unsure of whom to trust, he goes to senior Civil Servant Oliver Lacon and, acting as his link man, Guillam facilitates Smiley's secret return from retirement in order to conduct a surreptitious investigation into the identity of the mole.
Guillam is motivated not only by his loyalty to Smiley but also because, as a result of the Mole's actions, a network of secret agents he ran in the former French North Africa were identified and executed.
Reappearing in The Honourable Schoolboy, Guillam is brought into Smiley's "inner circle" in the "post-Fall" Circus, along with Connie Sachs and "China watcher" Doc di Salis, as they investigate a Soviet "gold seam" in Hong Kong.
By the time of the events in Smiley's People (the third book in the Karla trilogy) Guillam is married to a young Frenchwoman called Marie-Clare and is head of Circus operations in Paris, though this too is considered to be a minor post.