This study examines how population aging will shape a crucial aspect of mental health and social well-being - loneliness. Drawing on theories of demographic metabolism, United Nations' population estimates and projections, and survey data covering approximately 50% of the world's population aged 50 and above living in 27 countries, we estimate the role of population aging in shaping cross-national differences in loneliness from 1990 to 2050. We use survey data to estimate the prevalence of late middle age and older adult loneliness by age and sex, and then apply these rates to the evolving age and sex distributions of the populations. Our results highlight massive increases in loneliness at ages 50 and above with a tripling of the number of lonely adults in these age groups in our sample countries from 104.9 million in 1990 to 333.5 million in 2050, increasing variability across countries in the share of their populations composed of lonely adults 50 and above, and the feminization of global later life loneliness with an increasing share of lonely adults in these age ranges being women. These results illustrate the power of demographic modeling to advance understandings of national profiles of mental health and social well-being.