Background: Theory-based telephone counseling exercise (TCE) interventions can increase aerobic exercise behavior in cancer survivors. Few studies, however, assess intervention effects on social cognitive variables. Here, the authors examined changes in social cognitive variables from a TCE intervention based on the multi-process action control framework in hematologic cancer survivors. Methods: A total of 51 hematologic cancer survivors were randomized to weekly TCE (n = 26) or self-directed exercise (n = 25) for 12 weeks. Participants self-reported on demographic and cancer variables, as well as motivational, regulatory, and reflexive ratings pertaining to aerobic exercise at baseline and post-intervention. Results: Small-to-large between-group differences in all variables favored the TCE group. The most prominent effects were noted for differences in coping planning (adjusted mean between-group difference [MBGDadj] = 1.4, 95% confidence interval [CI], 0.7 to 2.2, d = 1.04), instrumental attitude (MBGDadj = 0.5, 95% CI, 0.1 to 1.0, d = 1.11), affective attitude (MBGDadj = 0.6, 95% CI, 0.1 to 1.2, d = 0.71), and perceived opportunity (MBGDadj = 0.4, 95% CI, −0.3 to 1.2, d = 0.50). Changes in coping planning (b = 24.98, β = 0.18, 95% CI, −0.03 to 0.56), perceived opportunity (b = 17.95, β = 0.13, 95% CI, 0.01 to 0.36), exercise identity (b = 17.43, β = 0.12, 95% CI, −0.05 to 0.41), and habit (b = 14.64, β = 0.10, 95% CI, −0.01 to 0.42) accounted for the largest mediating effects on aerobic exercise behavior. Conclusions: Multi-process action control framework-based TCE interventions may strengthen motivational, regulatory, and reflexive profiles that translate into increased aerobic exercise behavior in hematologic cancer survivors.