Apathy is the most common and disabling non-cognitive feature of dementia, affecting up to 90% of individuals over the disease course. Despite its prevalence, the underlying mechanisms of apathy remain elusive. This study aimed to investigate whether cognitive apathy and executive functioning have a shared cognitive and neural basis, in behavioural-variant frontotemporal dementia (bvFTD) and Alzheimer's disease (AD). Seventy-one participants (31 bvFTD, 17 AD and 23 controls) were assessed on a neuropsychological battery of executive tasks including the Zoo Map Test, Modified Six Elements Test, Tower Test and verbal fluency. The Dimensional Apathy Scale (DAS) was used to quantify cognitive apathy. Principal components analysis identified a single component underpinning performance on the neuropsychological tests, with both bvFTD and AD showing significantly reduced "planning ability" compared to controls. On the DAS, 74% of bvFTD patients and 59% of AD patients showed clinically significant cognitive apathy. Importantly, linear regression revealed that lower planning ability significantly predicted increased cognitive apathy, even after controlling for cognitive impairment and disease duration. Voxel-based morphometry analyses revealed that planning ability and cognitive apathy were both associated with atrophy of the right frontal pole and orbitofrontal cortex, as well as the thalamus and putamen. From a theoretical perspective, our results reveal a shared mechanism underpinning both cognitive apathy and planning deficits in bvFTD and AD. Clinically, this knowledge will help to improve the identification of apathy in clinical syndromes and inform targeted interventions to improve independence and wellbeing for those living with dementia.