Purpose: The experiences, skills, and internal resources that informal caregivers bring into their role may play a critical part in their mental health and well-being. This study examined how caregiver internal resources changed over a 10 year period, and how this was related to caregivers' well-being.; Methods: Data are from the Midlife in the United States (MIDUS) study, a national sample of adults, at two time points: 1995-1996 (T1) and 2004-2006 (T2). We identified subjects who reported being a caregiver at T2 and starting care after T1 (mean age = 56; 65% female). We examined internal resources: sense of control (personal mastery); primary and secondary control strategies (persistence in goal striving, positive reappraisal, and lowering expectations); and social support seeking, and psychological and subjective well-being. We evaluated how internal resources changed over time, and how these trajectories were associated with well-being at T2 using multivariable linear regressions.; Results: Most caregivers had stable levels of internal resources (between 4 and 13% showed an increase or decrease). Caregivers with increasing or high-stable levels of personal mastery had significantly better well-being scores on 6 out of 8 subscales compared with low-stable levels [effect sizes (ES) between 0.39 and 0.79]. Increasing persistence was associated with better personal growth and environmental mastery (ES = 0.96 and 0.91), and increasing and high-stable positive reappraisals were associated with better affect (ES = 0.63 and 0.48) compared with low-stable levels. Lowering aspirations and support seeking were not associated with well-being outcomes.; Conclusions: Practices or interventions that support or improve internal resources could potentially improve caregiver well-being.