Loneliness and social network size have been found to be predictors of mortality in older adults. The objective of this study was to investigate whether loneliness and small social network size are associated with an increased mortality risk and to review the evidence for either network size, or loneliness that constitutes the higher mortality risk. A systematic literature search was performed in PubMed, EMBASE and PsychInfo in January/February 2018 and March/April 2021. Studies that mentioned outcome data were included in the meta-analysis and coded using the Newcastle-Ottawa Quality Assessment Scale for Cohort Studies. The meta-analysis showed that both loneliness and small social network size are associated with mortality risk in older adults (Hazard Ratio 1.10 (95% Confidence Interval 1.06-1.14) for loneliness and 0.96 (95% Confidence Interval 0.93-0.99) for larger network size). Sensitivity analyses according to the Newcastle-Ottawa Quality Assessment Scale yielded varying results. Heterogeneity was large. In conclusion, both loneliness and small social network size in older adults are associated with increased mortality, although the effect size is small. Targeting subjective and objective aspects of older adults' social contacts should be on the agenda of preventive as well as personalized medicine. In order to be able to compare the association between loneliness and network size and mortality, more studies are needed that include both these risk factors. The online version contains supplementary material available at 10.1007/s10433-022-00740-z.