Objective Dementia with Lewy bodies (DLB) is associated with a range of cognitive and non‐cognitive symptoms. We aimed to identify if some of these symptoms might aid early diagnosis of Lewy body disease in cases of mild cognitive impairment (MCI). Methods Lewy body MCI (MCI‐LB; n = 36), Alzheimer's disease MCI (MCI‐AD; n = 21), DLB (n = 36), AD (n = 21) and control (n = 20) participants were recruited. An interview‐based questionnaire about the presence of symptoms thought to be associated with Lewy body disease was completed by participants with, where possible, their carer/relative. The prevalence of each symptom was compared between MCI‐LB and MCI‐AD and between established DLB and AD, and a symptom scale based on these findings was devised. Results Fluctuating concentration/attention; episodes of confusion; muscle rigidity; changes in hand‐writing, gait and posture; falls; drooling; weak voice; symptoms of REM sleep behaviour disorder (RBD) and misjudging objects were more common in MCI‐LB compared with MCI‐AD, and also in DLB compared with AD. Hyposmia, tremor, slowness and autonomic symptoms were not specific to Lewy body disease. REM sleep behaviour disorder and hyposmia were reported to develop several years prior to the onset of cognitive symptoms in Lewy body disease. A 10‐point symptom scale differentiated between MCI‐LB and MCI‐AD with a sensitivity of 83% and a specificity of 100%. Conclusions Drooling, misjudging objects and symptoms related to parkinsonism, fluctuating cognition and RBD may be the most characteristic symptoms of MCI‐LB. Slowness, tremor, autonomic symptoms and hyposmia are all common in MCI‐LB but are not specific to the disease. Copyright © 2017 John Wiley & Sons, Ltd.