He was first assigned as first officer of Global Community One, and was reassigned as first officer of the Condor 216 when GC One was destroyed in New York City in the outbreak of World War III. Mac was then promoted to Captain of the Condor 216 when former captain Rayford Steele fled from Global Community employ. He later recruits fellow believer Abdullah Smith to be the Condor's new first officer. Mac was then appointed to the position of captain of the Phoenix 216, with Smith as his first officer, after the Condor is destroyed in Johannesburg, South Africa.
On the night of the Wrath of the Lamb earthquake, McCullum is shocked into considering that the "ravings" of the Christian remnant might just hold water. Seeing the moon turn blood-red, he remembers the prophecy given coverage just a few days earlier in Global Community Weekly, the magazine edited by Cameron "Buck" Williams.
In a matter of days, McCullum becomes a Christian believer, and just in time to take Steele's place as captain of Global Community One, and custodian of the secret listening device installed in the plane by Steele's former boss, Earl Halliday, at Pan-Continental Airlines.
He is presumed dead by the GC after the crash of a Quasi Two off the coast of Israel during a GC air show. The Quasi was believed by the GC to be carrying Mac, Abdullah Smith, David Hassid, and Hannah Palemoon. In truth, they decided to flee the GC Headquarters Palace in New Babylon, Iraq before being forced to take the Mark of the Beast, and were not actually on the Two when it went down (McCullum was controlling it via remote from a air strip in Jordan).
Mac goes on to be an invaluable member of the Trib Force, even after his mole days are over. He helps organize and carry out Operation Eagle, and shortly after he leads a rescue mission in Greece to rescue George Sebastian. Shortly before the Battle of Armageddon, he leads yet another rescue mission into the heart of New Babylon to extract all believers inside the city before it is destroyed by fire from heaven.
In Glorious Appearing, Mac goes undercover in a decimated Jerusalem to try and rescue Buck Williams, only to find that the young man was already dead. Mac was among the few believers still alive to witness the return of Christ. In the Millennial Kingdom, he goes with Rayford's missionary group into Egypt and helps lead countless undecideds to salvation. Near the end of the Millennium, the Trib Force throws the aging Mac a thousanth birthday party. Mac is present at the final battle of Armageddon, where Christ defeats Satan and welcomes all believers into Paradise.