Self-Determination Theory (SDT) suggests that individuals with greater basic psychological needs satisfaction and autonomous motivation are more likely to adopt physical activity (PA). However, few longitudinal studies have examined these processes in exercise initiates outside the context of a formalized intervention. Further, an implicit yet relatively unexplored tenet of SDT is that when behaviors are closely connected to what is meaningful to individuals, those behaviors are more likely to be maintained. This study had two aims: (a) to examine SDT and a sense of meaning in life (MIL) in relation to PA in previously inactive exercise initiates, and (b) to determine whether daily measurement of MIL, mood, and PA impacts PA engagement. Upon joining a fitness center, participants (N = 160; M age = 43.3 years; 77% female) were recruited for a longitudinal study of PA adoption. Participants were randomized to a daily measurement condition (self-monitoring) where they reported daily MIL, mood, and PA or to an attention control condition. All participants self-reported their PA, basic psychological needs satisfaction, behavioral exercise regulation, and MIL at baseline, 4-weeks, and 12-weeks after starting an exercise program. Longitudinal path models were used to examine the relationships among study variables. Cross-sectionally, SDT constructs were largely related to PA in the expected directions. After accounting for SDT processes, baseline MIL was associated with greater PA at 4-weeks (p = .028). There were no differences in PA, SDT constructs, or MIL between the measurement self-monitoring and control groups. MIL predicted PA adoption above and beyond SDT predictors in previously inactive adults. Combined SDT and MIL intervention approaches may enhance PA adoption, and further research should explore their roles in PA adoption and maintenance.