Plastic mulch films contribute to better crop production. Concerns for lack of sustainable disposal methods for conventional polyethylene (PE) mulch led to development of biodegradable plastic mulches (BDMs) that can be soil-incorporated or composted after use. Environmental weathering of BDMs during crop growth reduces their mechanical strength and alters the molecular structure of their polymeric components. However, the impact of weathering on BDMs’ biodegradability is not fully understood. The biodegradability of agriculturally weathered and unweathered BDMs in soil and compost was compared using standardized laboratory tests (ASTM D5988 and D5338) using four BDMs (experimental polylactic acid and polyhydroxyalkanoate-based film [PLA/PHA] and three commercially available polybutyrate [PBAT]-based BDMs). In soil, biodegradation of weathered PLA/PHA was greater than its unweathered counterpart. For PBAT-based BDMs, the extent of biodegradation varied. A decrease of the weight-averaged molecular weight (Mw) of PBAT and PLA and thermostability of PLA, PHA, PBAT, and starch components was observed during biodegradation in the soil. The proportion of the minor components PHA and starch decreased during biodegradation, indicating preferential utilization of PHA over PLA and starch over PBAT by microbes. Bacterial abundance was significantly higher than fungal abundance in soil and was more prominent in soil adjacent to weathered than unweathered BDM treatments. Under composting conditions, unweathered PBAT-enriched mulches yielded higher CO2 evolution than their weathered counterpart. Together, these results suggest that environmental weathering enhances biodegradation of BDMs and mulch’s polymeric constituents also influence the microbial degradation, more so for bacterial than fungal communities.