Within self-determination theory, motivation has been treated as a unidimensional (autonomy continuum) and multidimensional (types of motivation) construct. We propose that this dual nature can be reconciled by distinguishing reasons for exerting effort from the psychological state, or ‘mindset’, experienced while engaging in task-relevant activities. Using the Multidimensional Work Motivation Scale (Gagné et al., European Journal of Work and Organizational Psychology 24:178–196, 2015) for employees (n = 444), and an adapted version for students (n = 656), we confirm that there are distinguishable reasons for exerting effort. Moreover, using measures of three motivational mindsets (experienced autonomy, external control, and motivation strength), we demonstrate that reasons vary along an autonomy continuum. However, latent profile analyses revealed that reasons combine in different ways, and that external reasons (e.g., social or material outcomes) are only associated with a mindset of external control when not accompanied by internal reasons. Outcomes were more favorable for autonomous versus externally controlled mindsets, and when motivation profiles were characterized by internal sources of regulation, even if accompanied by external reasons. Implications for the dimensionality and measurement of motivation are discussed.