Scholars and the media have long attributed Americans’ relatively high tolerance toward outcome inequality and low demand for redistribution to their belief in social mobility. However, previous research reports inconsistent findings on their mobility perceptions. While some studies suggest that Americans exhibit mobility optimism, other studies reveal that they are pessimistic about mobility prospects in the country. This research investigates Americans’ perceptions and misperceptions of social mobility from three aspects — measurements, sources, and consequences. Specifically, I conduct a nationally representative longitudinal survey experiment to examine (1) how measurements of perceived social mobility matter, (2) the potential sources of mobility misperceptions, and (3) if Americans’ attitudes toward outcome inequality and redistribution can be altered when they receive factual information about social mobility in the United States relative to Europe. My empirical results show that Americans perceive the bottom-to-top (“rags-to-riches”) upward mobility and relative mobility very differently. While the perception of the bottom-to-top mobility is highly polarized, Americans predominantly overestimate the intergenerational association of economic success. In addition, while an awareness of the European nobility system significantly increases respondents’ perceived mobility chances in the United States, an awareness of the contemporary redistributive regime difference between America and Europe increases their pessimism. Furthermore, after being informed of the reality, respondents’ attitudes toward outcome inequality and redistribution become more polarized if social mobility is regarded as the bottom-to-top upward mobility and become more converged if social mobility is referred to as relative mobility. The treatment effects persist for at least one week. These findings highlight the importance of distinguishing different dimensions of social mobility when studying individuals’ mobility perceptions.