Catalytic and non-catalytic microwave-assisted co-pyrolysis of biomass with plastics was performed to understand the interactions. An ex-situ configuration was adopted for performing catalytic co-pyrolysis experiments with ZSM-5 as a catalyst. Co-pyrolysis promoted cracking of vapors resulting in enhanced gas yields. ZSM-5 further enhanced the secondary cracking which resulted in low oil yields. The oil fraction collected from the pyrolysis of plastics was rich in hydrocarbons, whereas biomass pyrolysis led to the formation of oxygenated compounds in the oil. A plausible reaction mechanism scheme is proposed to understand the formation of major pyrolysis products via different pathways during different pyrolysis processes investigated. Also, a new parameter, the pyrolysis index is introduced to understand the pyrolysis intensity by utilizing the feedstock conversion, pyrolysis time, heating value, mass of feedstock, and energy consumption. The value of the pyrolysis index was found to be higher for plastics pyrolysis than biomass pyrolysis. Co-pyrolysis further increased the pyrolysis index due to the synergistic interactions.