|Family||Sgt. Lilly - Daughter (Deceased)|
|First Appearance||"Warren's Dream"|
|Last Appearance||"The Vanishing"|
|Death Episode||"The Vanishing"|
|Cause of Death||Bitten by Sgt. Czarnecki. Out of mercy by Sgt. Lilly|
|Series lifespan||"Warren's Dream" to "The Vanishing"|
Nothing is known about Lt. Mueller prior to the apocalypse, other than that he was an officer in the U.S. Marine Corps. He has a daughter named Lilley.
Lt. Mueller is seen consoling refugees at the camp.
During a search for the missing convoy, the group come under attack. Sgt. Czarnecki, a Marine under the lieutenant's command, is wounded and unable to be rescued; Lt. Mueller is forced to give the mercy order on his friend to prevent him from becoming a zombie. However, it doesn't kill the sergeant and the group retreats to the camp.
The zombified Sgt. Czarnecki is found by Lt. Mueller. Mueller shoots Czarnecki in the head, thinking he has given mercy to him. Mueller is incorrect and Czarnecki is seen getting up in the background while the lieutenant is looking away; Mueller is then killed off-screen. He later returns to the camp as a zombie and attacks the group. He is then killed permanently by Pvt. Lilley