In the season premiere "Acheron: Part I", Eugene, Ezekiel, Yumiko, and Princess are transported into a compound guarded by paramilitary troopers.
They come up with a plan to gain access to the radios secretly, but shortly after making contact with Rosita, they are cut off and arrested by Mercer.
In the third part of the season, as people riot after discovering Sebastian's crimes, many of them are shown to be calling for Mercer to take up leadership of the Commonwealth.
Rosita helps Mercer deal with a massive herd that threatens the Commonwealth itself while Negan arrives seeking entry to warn Carol and the others about Lance's rogue actions.
However, things go sideways on Founder's Day when a number of reanimated Commonwealth janitors, murdered by Lance's forces, attack, resulting in the death of Sebastian before Daryl, Judith and Mercer manage to put down all of the walkers.
Mercer begins gathering troops that feel the same way and makes contact with Princess who reveals the breakout of the people who had been disappeared by Pamela and Lance.
As a massive herd overruns the Commonwealth, Max and Princess break Mercer out and he helps them get into the Estates where Pamela has evacuated the key personnel, including the doctors that are needed to treat Judith's gunshot wound.
Ultimately, Colonel Vickers realizes that Daryl and Gabriel are right and she defers to Mercer who arrests Pamela for high crimes against the people of the Commonwealth and coordinates the successful operation to destroy the herd.
[4] Ron Hogan of Den of Geek reviewed the next episode "Acheron: Part II" and mentioned Mercer "who I can only call the Pumpkin King due to the color of his armor".
Club, Alex McLevy noted how the "hulking red stormtrooper" Mercer went to West Point, and granted asylum to Eugene and the others.
[7] Writing for Forbes, Erik Kain reviewed the episode "Out of the Ashes", notes how "Princess tries to distract Mercer which probably makes matters worse".