Importance: Rates of PTSD and major depression are high among paramedics. Objective: To evaluate the efficacy of a cognitive resilience training programme for reducing the development of PTSD and major depression among early career paramedics compared to psychoeducation and standard practice. Design, Setting, Participants: Paramedics training at 15 universities across England participated between October 2017 to December 2024 with 12 and 24 month follow-up; 12-month outcomes reported here. Intervention: Internet-delivered cognitive training in resilience (iCT-R) is a guided online intervention that utilises cognitive therapy tools to target predictors of PTSD and MDD identified in prospective research with paramedics. It consists of 6 modules delivered over 6 weeks with 6 monthly top-up sessions delivered by email. Internet-delivered psychoeducation is a supported online psychoeducation intervention, covering 6 topics delivered one per week with 6 monthly top-up sessions delivered by email. Participants were randomised to iCT-R or online psychoeducation or standard practice. Main Outcomes and Measures: The primary outcome was rate of PTSD and MDD at one-year follow-up, assessed by independent assessors blinded to intervention using the Structured Clinical Interview for DSM-5. Secondary outcomes included measures of PTSD and depression symptom severity, resilience, rumination, anxiety, psychological distress, and well-being. Intent-to-treat analyses were conducted, with the primary outcome analysed using mixed-effects logistic regression. Results: Of 570 student paramedics enrolled, 195 were allocated to iCT-R, 197 to psychoeducation, and 178 to standard practice. Mean age of participants was 23 years, 35% were men, and most were White British. For participants allocated to iCT-R, the odds of meeting criteria for PTSD or MDD at 12 months were significantly lower compared to psychoeducation (OR=0.20, 95% CI [0.05, 0.73]) and standard practice (OR=0.25 [0.07, 0.97]. Providing iCT-R training to 18-24 paramedic trainees (number needed to treat) would prevent one case of PTSD or MDD. Participants receiving iCT-R were approximately 5 times less likely to develop PTSD or MDD at one-year follow-up compared to psychoeducation and 4 times less likely compared to standard practice. Conclusions and Relevance: Internet-delivered cognitive training in resilience appears to decrease the likelihood of developing PTSD and MDD in early career paramedics. Trial Registration: ISRCTN16493616; Registered prospectively.