Schizophrenia and bipolar disorder share genetic risk, brain vulnerability, and clinical symptoms. The ZNF804A risk variant, rs1344706, confers susceptibility for both disorders. This study aimed to identify neural mechanisms common to both schizophrenia and bipolar disorder through this variant's potential effects on cortical thickness, white matter tract integrity, and cognitive function. Imaging, genetics, and cognitive measures were ascertained in 62 healthy adults aged between 18 and 59 years. High-resolution multimodal MRI/DTI imaging was used to measure cortical thickness and major frontotemporal and interhemispheric white matter tracts. The general linear model was used to examine the influence of the ZNF804A rs1344706 risk variant on cortical thickness, white matter tract integrity, and cognitive measures. Individuals homozygous for the risk variant ('A' allele) demonstrated reduced cortical gray matter thickness in the superior temporal gyrus, and in the anterior and posterior cingulate cortices compared with C-allele carriers. No effect of the risk variant on microstructural integrity of white matter tracts was found. Reduced attention control was found in risk allele homozygotes, aligning with findings in the anterior cingulate cortex. Our data provide a novel, genetically based neural risk mechanism for the major psychoses by effects of the ZNF804A risk variant on neural structures and cognitive function susceptible in both disorders. Our findings link genetic, imaging, and cognitive susceptibility relevant to both schizophrenia and bipolar disorder.