Objective: N -acetylcysteine (NAC) is a novel therapeutic agent with multiple mechanisms of action in the central nervous system and a favourable side effect profile. Clinical evidence indicates that adjunctive NAC may reduce the severity of depressive symptoms in individuals with major depressive disorder (MDD). Methods: A 12-week randomised controlled trial of 2,000 mg/day adjunctive NAC for MDD found no significant improvement at the primary endpoint (week 12) but did see improvements at the post-discontinuation interview (week 16). Within the context of patient-centered treatment, mixed-methods qualitative analysis was also included to explore factors that may determine individual responses to adjunctive NAC treatment. These data were drawn, under blinded conditions, from clinician notes recorded in the case report form. Using the DSM-5 symptom profile for MDD as the initial framework, themes were developed and explored. Frequencies were compared between placebo and NAC groups. Results: Per protocol analysis of individual themes across the six interviews revealed group differences in favour of NAC for overall depressive affect, optimism, relationships and reduced functional impairment. Conclusion This study provides further evidence for the utility of the mixed methods approach complimenting the primary findings using traditional quantitative analyses, as well as being able to capture additional, often more subtle, evidence of individual symptom-level change that reflects improvement in functional abilities in response to NAC supplementation. The use of mixed methods to explore outcomes from psychiatric studies should be considered in future to work towards improved patient-centred care and both confirm quantitative findings and generate novel hypotheses.