Markham is a sergeant in the United States Marine Corps and a member on the Atlantis expedition. Since he has the Ancient Technology Activation gene, he was several times used to pilot a Puddle Jumper.
When the Atlantis Expedition decided to examine the Wraith homeworld a second time, both Markham and Sgt. Stackhouse accompanied Major John Sheppard's team. When Sheppard was bitten by an Iratus bug, it was Markham who piloted the damaged Jumper back to Atlantis. When the engines of the Jumper didn't retract while flying through the Stargate, Markham and Stackhouse became trapped while dematerialized on the other side of the event horizon. The Jumper, however, was in time able to pass through the gate, preventing both Marines from dying. (SGA: "Thirty-Eight Minutes")
Several months later, Markham piloted a Puddle Jumper to another planet in the Lantean system, when both Major John Sheppard and Dr. Rodney McKay were trapped on it. He eventually killed the Wraith survivor on the planet with a Drone weapon. (SGA: "The Defiant One")
When Atlantis detected a single Wraith Dart flying en route to the city, Markham was among the people piloting a Jumper to intercept the dart. Unfortunately, his Jumper was shot down, the impact of the crash killing both Markham and Smith instantly. (SGA: "The Brotherhood")
A few days later, while recording messages to be sent back to Earth, Weir sent a message to Markham's family informing them of his death. Weir tells Markham's family that he died with honor and that his death may have saved others. (SGA: "Letters from Pegasus")
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