Cerebral Autosomal Dominant Arteriopathy with Subcortical Infarcts and Leukoencephalopathy (CADASIL) is a rare monogenic disorder caused by mutations in the NOTCH3 gene. The clinical features are primarily neurological, which include recurrent transient ischaemic attacks, strokes, and migraines. However, psychiatric manifestations which mainly include mood disturbances have also been reported in CADASIL. Manic symptoms and bipolar disorders are rarely documented in CADASIL and existing reports generally lack detailed descriptions of the psychiatric evaluation. We discuss a case of Bipolar Affective Disorder (BD) in a British woman with a family history of CADASIL. This case provides insight into the diagnosis and management of BD as well as the possible underlying aetiologies that should be considered. The similarities between BD and CADASIL in terms of imaging, genetic, and therapeutic aspects raise the possibility of common dysfunctional pathways. BD in CADASIL may warrant greater consideration by both psychiatrists as well as non-psychiatric specialists and further studies are required to understand the pathological significance.