Simple Summary In the current era of data explosion, the use of genetic information is increasingly being applied across numerous biological questions. One application has been to develop more robust evolutionary frameworks. Such well-resolved phylogenetic relationships are currently lacking from many of the basal branches of diversity-rich taxa. This is most pronounced at the base of the thentredinoid, especially Athalia. This study reviews earlier comparative morphological studies and advances in phylogenetic studies based on morphological characters and short sequence fragments, using both mitochondrial and nuclear genetic sequences as well as genomic structural evidence to define the family Athaliidae in several dimensions and clarify its phylogenetic position. As a result, Athaliidae no longer belongs to the Tenthredinidae but is independent and distant from it. This study clarifies a hurdle to solve the scientific problem of hymenopteran evolution. The systematic status of the genus Athalia and related genera is a perennial controversy in sawfly taxonomy. Several authors have hypothesized that the placement of Athalia within the Tenthredinidae is artificial, but no studies have focused on this topic. If the hypothesis that Athalia does not belong to Tenthredinidae can be supported, the taxonomic framework of Tenthredinoidea needs revision. We present a comprehensive phylogenomic study of Tenthredinoidae, focusing on the positions of Athalia and related genera by sampling 80 representatives mainly of the Tenthredinoidea, including Heptamelinae and Blasticotomidae. Our phylogenetic reconstructions based on nuclear genes and mitochondrial (mt) sequences support Athalia and related genera as a distinct clade sister to Tenthredinidae + (Cimbicidae + Diprionidae). A comparison of symphytan mitochondrial genomes reveals an innovative gene rearrangement pattern in Athaliidae, in which Dentathalia demonstrates a more ancestral pattern than Athalia and Hypsathalia. The lineage specificity of mt rRNA secondary structures also provides sufficient support to consider Athaliidae as a separate family. In summary, the phylogeny and genomic structural changes unanimously support the taxonomic treatment of Athaliidae as a family and the re-establishment of Dentathalia as a valid genus.