There is growing evidence that loneliness is associated with mild cognitive impairment (MCI) and dementia. However, the extent of this association remains unclear. A systematic review and meta-analysis of longitudinal studies examining this association was conducted. Six electronic databases were searched from inception to November 15th 2018. A random-effects meta-analysis was performed to obtain pooled estimates and 95% CIs. Studies were also assessed for heterogeneity, methodological quality and publication bias. A total of 4270 hits were retrieved based on the initial search strategy and ten studies met the eligibility criteria involving 37339 individuals (mean age from 64.9 to 83.1 years). Variation between studies was present for the measurement of loneliness as well as for the case ascertainment of MCI and dementia. Loneliness was positively associated with increased risk of dementia (overall RR = 1.26; 95% CI = 1.14, 1.40; n = 8). Due to lack of sufficient data, we could not explore the association between loneliness and risk of MCI through a meta-analysis, but limited evidence suggests a potential effect of loneliness on MCI. A further understanding of the deleterious effects of loneliness on MCI and dementia may assist the design of environmental and psychological interventions to prevent or delay the onset of these neuropsychiatric conditions.