It is unknown if Sumner had any family who was still alive, and virtually nothing is known about his personal life. What is known was that he was very good friends with Colonel Dillon Everett. (SGA: "Letters from Pegasus", "The Siege, Part 2")
He objected to Major John Sheppard's inclusion in the Atlantis expedition, stating that he disliked any officer who failed to follow the proper chain of command. After Atlantis was discovered to be underwater and slowly flooding, Sumner led the reconnaissance team that made first contact with the Athosians. Though he allowed Sheppard to attempt to establish a rapport with them, he paid less attention to the people than to any potential technology or suitable evacuation site that might be found; this led him to disregard the Athosians' warning against exploring the ruins of an Ancient city there. Teyla Emmagan observed that Sumner "looks through me as if I were not there.". In short, his attitude was very similar to that of Jack O'Neil in the original film, seeing the eggheads as an obstacle to his military operation, and a potential threat to his command and possibly his authority.
When the Wraith culled the Athosians, Sumner was captured along with some of the Athosians. When the Wraith came to collect an Athosian prisoner, Sumner attempted to offer himself to save the Athosian, but the Wraith refused the initial offer. After killing the Athosian, they returned to the cell and brought Sumner before the Wraith Keeper, who attempted to interrogate him about Earth. He refused to tell her the Stargate address, and, noting his surprising strength of will, the Keeper fed upon him, commenting that she had not tasted such strength in a very long time. Although Sheppard tried to rescue him, he did not arrive until Sumner appeared to have aged by several decades, with the Keeper still feeding on him.
When the Keeper proved resistant to gunfire, Sheppard shot Sumner instead, believing it was what Sumner wanted him to do with Sumner later dying instantly as a result. This made Sumner the first ever Tau'ri casualty of the Tau'ri-Wraith war. After returning to Atlantis, the authority rested onto Sheppard until it became official in 2005 by Stargate Command, and some persuasion by Weir. (SGA: "Rising", "The Intruder")
Sumner's death would be a major source of contention between Sheppard and both Colonel Dillon Everett, who had considered Sumner a very good friend, and Colonel Steven Caldwell, although both eventually acknowledged that he had done so for the right reasons. For his part, Sheppard spoke highly of Sumner in a message back to Earth, saying he had borne the burden of a leader well and that in light of the looming Wraith assault, he wished Sumner was still with them. (SGA: "Letters from Pegasus", "The Siege, Part 2", "Runner")
- In an alternate timeline, the expedition arrived in Atlantis, where he discovered it to be underwater. However, in that timeline, the city's failsafes did not exist and the city took on water. Sumner was one of many who drowned while attempting to leave the city. (SGA: "Before I Sleep")