At least some animal species can generate neurons from mesoderm or endoderm, but the underlying mechanisms remain unknown. We screened for C. elegans mutants in which the presumptive mesoderm-derived I4 neuron adopts a muscle-like cell fate. From this screen, we identified HLH-3, the C. elegans homolog of a mammalian proneural protein (Ascl1) used for in vitro neuronal reprogramming, as required for efficient I4 neurogenesis. We discovered that the CDK-8 Mediator kinase module acts together with a second proneural protein, HLH-2, and in parallel to HLH-3 to promote I4 neurogenesis. Genetic analysis revealed that CDK-8 most likely promotes I4 neurogenesis by inhibiting the CDK-7/CYH-1 (CDK7/cyclin H) kinase module of the transcription initiation factor TFIIH. Ectopic expression of HLH-2 and HLH-3 together promoted expression of neuronal features in non-neuronal cells. These findings reveal that the Mediator CDK8 kinase module can promote non-ectodermal neurogenesis and suggest that inhibiting CDK7/cyclin H might similarly promote neurogenesis.