Plasmids facilitate antimicrobial resistance (AMR) gene spread via horizontal gene transfer, yet the mobility of genes in wastewater treatment plant (WWTP) resistomes remains unclear. We sequenced 173 circularised plasmids transferred from WWTP effluent into Escherichia coli and characterised their genetic content. Multiple multidrug-resistant plasmids were identified, with a significant number of mega-plasmids (>100 kb). Almost all plasmids detected existed with other plasmids i.e. as communities rather than lone entities. These plasmid communities enabled non-AMR plasmids to survive antimicrobial selection by co-existing with resistant partners. Our data demonstrates the highly variable nature of plasmids in addition to their capacity to carry mobile elements and genes within these highly variable regions. The impact of these variations on plasmid ecology, persistence, and transfer requires further investigation. Plasmid communities warrant exploration across biomes, as many non-resistant plasmids escape elimination by co-existing with AMR plasmids in the same bacterial host, representing a previously unrecognised survival strategy.