Background: Measurements of the lexical-semantic ERP component N400 have been used to monitor neurophysiological change associated with repetitive transcranial magnetic simulation (rTMS) in persons with chronic non-fluent aphasia. Combined with reported favourable behavioural language changes, research evidence has informed the potential of rTMS to assist brain reorganisation patterns in persons with chronic language impairment. Aims: The present study aimed to provide evidence regarding the longitudinal effects of rTMS on N400, an ERP measure of lexical-semantic performance in persons with non-fluent aphasia. Further, it aimed to provide a comparison of N400 responses between the active and placebo group across semantically congruent and incongruent ERP conditions. Methods & Procedures: A total of 12 persons with aphasia were randomly assigned to receive active stimulation (n = 6) or placebo stimulation (n = 6). ERP measures from four centro-parietal electrodes were taken at baseline, 1 week, 2 months, 8 months, and 12 months post stimulation. Outcomes & Results: Significant interactions of group × time were identified for task performance between the active and placebo groups, where active performance was higher. Significant interaction effects of group × time × condition, for some electrodes across ERP measures of mean amplitude, peak amplitude, and area were found. Active group N400 responses improved (increased negativity) over 12 months, compared to the placebo group responses. Conclusions: The results demonstrated that modulation of N400 measures up to 12 months post rTMS for the active stimulation group was present. Electrophysiological changes were accompanied by improvements in anomaly detection over time. Overall, these findings highlight that low-frequency rTMS has the capacity to modulate lexical-semantic processing over time as evidenced via ERP measures.