He appears sitting at Detective Mike Anderson's desk, handling paperwork when Beth Dorsey arrives her attempt to release Wormwood, as part of The Wormwood Tableau. Mills instructs her to fill out a sheet in order to provide her "information" on the Doomsday Killer but she claims that she really needs to see Lieutenant Debra Morgan (as instructed by Travis Marshall). She takes a seat and sits there across from Mills for the time being.
When Deb does finally arrive and she gets a moment to herself in the office, Officer Mills comes to her door stating that a woman is here to see her regarding information about Wormwood. She tells Mills to go ahead and send her in, though he is unknowing of the fact that she intends to kill as many people as she can. Dexter Morgan is able to recognize her as relation to Steve Dorsey and as Beth activates Wormwood, he pushes her into an Interview room where the gas kills only her and Debra evacuates everyone, including Mills.
He has yet to be seen again after this point.