A prominent, hotly debated idea—the “ego depletion” phenomenon—suggests that engaging in effortful, demanding tasks leads to poorer subsequent self-control performance. Several theories seek to explain the emergence of ego depletion effects. The two most prominent ones are the strength model of self-control (Baumeister & Vohs, 2016) and the process model of self-control (Inzlicht & Schmeichel, 2012). Predictions of these models are predominantly identical on the behavioral level. The models’ predictions differ, however, on the level of invested mental effort. The present pre-registered study (N = 179) contrasted these competing predictions combining an established moderator counteracting ego depletion effects (i.e., self-affirmation) and psychophysiological indicators of mental effort (i.e., systolic blood pressure and preejection period). Our data provide moderate evidence for ego depletion—decrements in self-control performance after a high- versus low-demanding task in the nonaffirmed conditions. Self-affirmation had an unexpected effect: Contrary to previous research, self-affirmed participants performed similarly poorly as participants in the high demand + nonaffirmed condition. Although this unexpected finding limited the ability to contrast competing model predictions, it points to hitherto unknown effects of self-affirmation on self-control performance. Systolic blood pressure reactivity emerged as a valid indicator of invested mental effort, but the data show no sign of disengagement after a high-demanding task predicted by the process (but not the strength) model. We explore systolic blood pressure progression across the sequential task paradigm, suggest a testable account for the effects of self-affirmation on self-control performance, and discuss theoretical implications of the results for the two competing models.