Background: Deciphering hypoplastic left heart syndrome (HLHS) pathogenesis is confounded by its genetic heterogeneity and oligogenic underpinnings. Methods: Whole genome sequences were analyzed by 3 independent strategies to identify HLHS gene candidates, ranked by variant, gene, and disease-level metrics. Results: First, a genome-wide association study of 174 cases and 853 controls revealed suggestive association with a MYO18B intron 33 variant (rs2269628-G; frequency=0.55 versus 0.39; OR, 1.97 [95% CI, 1.54–2.52]; P =6.70×10 ^{− 8} ). Second, transmission disequilibrium testing of 161 HLHS proband-parent trios revealed overrepresentation of a MYO18B intron 42 variant (rs73154186-A; frequency=0.05; OR, 24 [95% CI, 3.2–177.4]; P =4.23×10 ^{− 6} ). Third, rare, predicted-damaging variants were filtered in 2 multiplex families. In 141H, 2 fifth-degree relatives with HLHS shared a paternally-inherited MYO5A missense variant (p.Arg801Trp; frequency=0.00003; combined annotation-dependent depletion score=29), each with a maternally-inherited or de novo candidate modifier variant in a MYO5A-interacting conventional myosin. In 442H, a HLHS proband was compound heterozygous for MYO15A variants—a maternally-inherited pathogenic stop-gain variant co-segregating with tetralogy of Fallot and bicuspid aortic valve in maternal relatives (p.Tyr2819Ter; frequency=0.00003) and a paternally-inherited intronic variant altering a canonical transcription factor binding site (rs1277068603; frequency=0.00001; position weight matrix score=0.98). Conclusions: Collectively, these findings suggest that common and rare alleles within unconventional myosin genes are associated with HLHS susceptibility. The identified candidate MYO18B regulates cardiac sarcomerogenesis, supporting the hypothesis of intrinsic myogenic perturbation in arrested left heart development.