This paper concerns the study of focused wave groups in mixed sea states. These focused wave events are considered representative of the largest events that occur in a random sea state. A fully non-linear Higher-Order Spectral (HOS) solver was adopted to simulate the wave groups and investigate the physical mechanisms that govern the evolution of these events. For the near breaking uni-directional wave groups in deep-water conditions considered, it was seen that irrespective of the relative strength of the swell and wind component, the maximum crests obtained were much larger than those predicted by the commonly adopted linear and second-order solutions. Further analysis revealed that the third-order resonant interactions that alter the underlying free wave components are responsible for these larger crests. However, the most important parameter that determines the amplification was found to be bandwidth; the smaller the bandwidth, the larger the resonant interactions and hence the larger the crest elevation obtained. More importantly, it was found that for the mixed seas, once steepness and bandwidth are taken into consideration, the level of nonlinear amplification of the crests was independent of the shape of the underlying wave spectrum. © 2023, The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd.