In the age of misinformation, conspiracy theories can have far-reaching consequences for individuals and society. Social and emotional experiences throughout the life course, such as loneliness, may be associated with a tendency to hold conspiracist worldviews. Here, we present results from a population-based sample of Norwegians followed for almost three decades, from adolescence into midlife (N = 2215). We examine participants’ life trajectories of loneliness using latent growth curve modeling. We show that people reporting high levels of loneliness in adolescence, and those who experience increasing loneliness over the life course, are more likely to endorse conspiracy worldviews in midlife.