Cognitive reappraisal, in which an individual alters the emotional meaning of a stimulus, is an effective emotion regulation strategy that can decrease an individual’s experience of negative emotion. This study sought to use the late positive potential (LPP), an event-related potential (ERP) component that has been shown to measure neural activity during emotional processing and regulation, to evaluate the effectiveness of different reappraisal strategies. The study’s aims were to compare self-generated reappraisal strategies, in which subjects were required to generate their own reappraisals, and externally provided reappraisals, in which descriptions that were negative or neutral were provided before viewing the image. Moreover, we looked at the sustained effects of these strategies on behavioral and brain measures of negative affect. Thirty-eight participants were shown images containing negative content while EEG data was recorded. Results revealed regulation effects on several early LPP components, with both types of reappraisals affecting amplitude for these peaks differently. Also, externally provided reappraisals led to larger passive task effects than self-generated reappraisal cues. Subjective responses showed a similar prolonged reduction in negative affect for both tasks despite pronounced differences in EEG activity. Externally provided reappraisals are more effective than self-generated ones, altering peak amplitudes and reducing the impact of negative images during passive viewing where these effects are not as distinct during the emotion regulation task. These findings have therapeutic implications, favoring guided regulation methods over self-reappraisal for sustained reappraisal effects.