Objective: The aim of this study was to test the effectiveness of a tailored quitline tobacco treatment (‘Quitlink’) among people receiving support for mental health conditions. Methods: We employed a prospective, cluster-randomised, open, blinded endpoint design to compare a control condition to our ‘Quitlink’ intervention. Both conditions received a brief intervention delivered by a peer researcher. Control participants received no further intervention. Quitlink participants were referred to a tailored 8-week quitline intervention delivered by dedicated Quitline counsellors plus combination nicotine replacement therapy. The primary outcome was self-reported 6 months continuous abstinence from end of treatment (8 months from baseline). Secondary outcomes included additional smoking outcomes, mental health symptoms, substance use and quality of life. A within-trial economic evaluation was conducted. Results: In total, 110 participants were recruited over 26 months and 91 had confirmed outcomes at 8 months post baseline. There was a difference in self-reported prolonged abstinence at 8-month follow-up between Quitlink (16%, n = 6) and control (2%, n = 1) conditions, which was not statistically significant (OR = 8.33 [0.52, 132.09] p = 0.131 available case). There was a significant difference in favour of the Quitlink condition on 7-day point prevalence at 2 months (OR = 8.06 [1.27, 51.00] p = 0.027 available case). Quitlink costs AU$9231 per additional quit achieved. Conclusion: The Quitlink intervention did not result in significantly higher rates of prolonged abstinence at 8 months post baseline. However, engagement rates and satisfaction with the ‘Quitlink’ intervention were high. While underpowered, the Quitlink intervention shows promise. A powered trial to determine its effectiveness for improving long-term cessation is warranted.