Antibiotics exert selective pressures on clinically relevant antibiotic resistance. It is critical to understand how antibiotic resistance evolves in environmental microbes exposed to subinhibitory concentrations of antibiotics and whether evolutionary dynamics and emergence of resistance are predictable. In this study, isolated from wastewater activated sludge were subcultured in a medium containing 10 ng/mL cefepime for 40 days (∼300 generations). Stepwise mutations were accumulated, leading to an ultimate 200-fold increase in the minimum inhibitory concentration (MIC) of cefepime. Early stage mutation in DNA polymerase-encoding gene played an important role in antibiotic resistance evolution. Diverse resistance mechanisms were employed and validated experimentally, including increased efflux, biofilm formation, reduced antibiotic uptake, and drug inactivation. The cefepime minimal selective concentrations (MSCs) and relative fitness of susceptible, intermediate, and resistant mutants were determined. Agent-based modeling of the modified Moran process enabled simulations of resistance evolution and predictions of the emergence time and frequency of resistant mutants. The unraveled cefepime resistance mechanisms could be employed by broader bacteria, and the newly developed model is applicable to the predictions of general resistance evolution. The improved knowledge facilitates the assessment, prediction, and mitigation of antibiotic resistance progression in antibiotic-polluted environments.