Objective The objective of the study is to report a systematic review of what is currently known about the experience of cancer treatment and cancer treatment in adults with dementia. Methods The analytic plan and inclusion/exclusion criteria were specified in advance of the search process in a protocol. Searches were conducted in MEDLINE, CINAHL, PsycINFO and the Cochrane Library for publications about people with cancer and a pre‐existing dementia. Limits were English language; 2000 to 12/2015; adults; >18 years old. The search identified 5214 titles and abstracts that were assessed against eligibility criteria and 101 were selected for full‐text examination by two researchers who agreed inclusion of nine papers, extracted data independently then conducted a content analysis and narrative synthesis. Results Nine studies conducted in four resource rich countries were included in the review. These studies evidence that when compared with other cancer patients, those with dementia are diagnosed at a later stage, receive less treatment, are more likely to experience complications from treatment and have poorer survival. The experience of supportive care and preferences of people with dementia receiving cancer services and cancer treatment have not been investigated. Research into how the cancer team manage the particular needs of people with dementia and their family members has been limited to one study that reported how a cancer team managed the particular needs of seven people with dementia. Conclusion Further work is needed to establish practice guidelines for the management of cancer in people with dementia. Copyright © 2016 John Wiley & Sons, Ltd.