Young onset dementia (YOD) is associated with significant costs and burden, but its cause is poorly understood. The aim of this review was to determine whether environmental and lifestyle factors are associated with risk for non-autosomal dominant degenerative and vascular YOD. Academic databases were searched to March 2015 for studies assessing the impact of modifiable factors (e.g. education, cardiovascular illness, psychiatric illness, alcohol use) in participants under 65 years at symptom onset. Cardiovascular illness, traumatic brain injury, psychiatric illness, heavy alcohol use and estrogen-related factors were identified as potential risk factors for YOD. Evidence for education, childhood development, smoking and heavy metal exposure was inconsistent or of poor quality. A dose–response relationship was found between cumulative and/or increasing severity of exposure and risk for YOD. Environmental and lifestyle risk factors may be relevant to YOD, particularly with severe or cumulative exposure. More high quality research is required to confirm which factors confer risk and when.