Simple Summary Tenthredinidae is the most speciose family of the paraphyletic ancestral grade Symphyta, including mainly phytophagous lineages. The subfamilial classification of this family has long been problematic with respect to their monophyly and/or phylogenetic placements. This article reports four complete sawfly mitogenomes of Cladiucha punctata, C. magnoliae, Megabeleses magnoliae, and M. liriodendrovorax for the first time. To investigate the mitogenome characteristics of Tenthredinidae, we also compare them with the previously reported tenthredinid mitogenomes. To explore the phylogenetic placements of these four species within this ecologically and economically important sawfly family, we perform the mitophylogenomics reconstruction and divergence time estimation analyses using a mitogenome dataset of 45 species of the superfamily Tenthredinoidea. Although these newly reported four species have currently classified in the subfamily of Allantinae, the obtained tree topology reveals the sister-group placement of Cladiucha and Megabeleses outside of Blennocampinae, Heterarthrinae, Tenthredininae, and the rest of Allantinae as a distinct lineage. In conjunction with the occurrence of morphological and molecular synapomorphic characters, apparent matches the reported divergence times of their host plants from Magnoliaceae-supporting the proposal of Megabelesesinae as a subfamily. This study represents a broad framework and valuable information for future research on this small subfamily. Tenthredinidae represents one of the external feeders of the most diverse superfamily, Tenthredinoidea, with diverse host plant utilization. In this study, four complete mitochondrial genomes (mitogenomes), those of Cladiucha punctata, Cladiucha magnoliae, Megabeleses magnoliae, and Megabeleses liriodendrovorax, are newly sequenced and comparatively analyzed with previously reported tenthredinid mitogenomes. The close investigation of mitogenomes and the phylogeny of Tenthredinidae leads us to the following conclusions: The subfamilial relationships and phylogenetic placements within Tenthredinidae are mostly found to be similar to the previously suggested phylogenies. However, the present phylogeny supports the monophyly of Megabelesesinae as a subfamily, with the sister-group placement of Cladiucha and Megabeleses outside of Allantinae. The occurrence of the same type of tRNA rearrangements (MQI and ANS1ERF) in the mitogenomes of Megabelesesinae species and the presence of apomorphic morphological characters also provide robust evidence for this new subfamily. The divergence and diversification times of the subfamilies appear to be directly related to colonization of the flowering plants following the Early Cretaceous. The origin time and diversification patterns of Megabelesesinae were also well matched with the divergence times of their host plants from Magnoliaceae.