Little is known about Mercer's life prior to or as the outbreak began, other than he served as an Marine and was on leave before the apocalypse started.
At some point in the apocalypse, Merce encountered a network of communities called the Commonwealth which he subsequently joined in the rank of Chief "Officer", while also acting as a personal bodyguard for the Miltons.
Mercer is watching over Sebastian who is with Kayla. Once walkers got too close to them, Mercer saves them, only for Kayla to yell and walk back to the Commonwealth and Sebastian yell at Mercer for ruining his date. Mercer is later talked down to by Sebastian's mother Pamela. Later Mercer is talking to George about over throwing Pamela, to which Siddiq overhears.
Mercer later disarms Juanita after she gets into a fight with a few officers. Sebastian wants the survivors banished for threatening him, but Mercer points out Sebastian provoked them, telling him that he can't go around slapping people for no reason. Sebastian then slaps Mercer, but is quick to retreat when he sees that he's angered the officer.
This list shows the victims Mercer has killed:
- Numerous counts of zombies and unnamed people
Mercer is Sebastian's personal bodyguard and the two have a very poor relationship. Sebastian is verbally abusive to Mercer, despite Mercer saving him from numerous counts of walkers which resulted in him ruining Sebastian's date with Kayla. Mercer likewise hates Sebastian strongly, but holds his tongue when around him. It's possibly that Sebastian sees Mercer as well as the other solders as expendable.
Pamela and Mercer have a complicated relationship. While Mercer is seemingly loyal to her and would do anything to protect her, he also hides a searing resentment for her as a leader, as she looks down on him and seems to see him as more of a tool than a person.
Mercer and George have a good relationship and often confide in each other. They go as far as conspire to overthrow Governor Milton, though they claim they are just blowing off steam when doing so.
Mercer and Princess are on good terms. She is romantically attracted to Mercer, when he saves her from a herd of walkers she kisses him.
Later on, after arriving at the Commonwealth, Juanita and Mercer have sex. Their conversation afterwards shows they feel something for each other. Mercer mentions that he really likes listening to Juanita talk and that it's definitely not a casual thing. They kiss and Juanita says that her life is getting better every day now, to which Mercer smiles happily in response.
Mercer was initially hesitant around Rick, but did confide in him that he was glad he now has an actual purpose in life after the apocalypse occurred. He later overhears Rick and Dwight interacting when the latter is in a holding cell and notices that Rick has the qualities the Commonwealth should have and views him as a more capable leader than Pamela.