Wastewater treatment plants (WWTPs) are important interfaces for the persistence and dissemination of antimicrobial resistance genes (ARGs) in the environment. This study investigated colistin resistance and the presence of mobile colistin resistance (mcr) genes in Enterobacterales isolated from a poultry slaughterhouse WWTP in Brazil. Samples were collected from raw sewage, an equalization tank, and treated effluent. A total of 27 Enterobacter spp. isolates were identified, of which 70.4% showed resistance to colistin (MIC range: 2 to ≥512 mg/L). PCR screening detected mcr-1 in two isolates and mcr-10 in three isolates distributed across all treatment stages, including the final effluent. Whole-genome sequencing of a representative isolate from treated effluent identified Enterobacter vonholyi ST3343, carrying a plasmid-borne mcr-10 gene on an ~107 kb IncFII(Yp) plasmid, along with additional resistance determinants. Phylogenetic analysis supported the classification of this gene as a novel allele, mcr-10.6. The persistence of a clonal lineage harboring mcr-10.6 throughout the treatment system indicates that conventional wastewater treatment may not effectively eliminate clinically relevant ARGs. These findings highlight treated effluent as a potential route for environmental dissemination of colistin resistance and reinforce the need for improved monitoring and mitigation strategies within a One Health framework.