Family members remain the main care providers for the increasing numbers of people with dementia, and often become depressed or anxious. In an implementation research project, we aimed to widen access to Strategies for RelaTives (START), a clinically and cost-effective intervention for the mental health of family carers, by laying the foundations for its implementation in the third sector. We used the integrated Promoting Action on Research Implementation in Health Services (i-PARIHS) framework to guide implementation of START, a manual-based, individually-delivered, multicomponent eight-session coping strategy intervention. We interviewed a maximum variation sample of twenty-seven stakeholders from the English Alzheimer's Society (AS), about possible difficulties in management, training, and delivery of START. We trained and supervised three AS dementia support workers in different locations, to each deliver START to three family carers. Two researchers independently coded pre-intervention interviews for themes. We assessed intervention feasibility through monitoring delivery fidelity, rating audio-recordings from 1-5 (5 being high) and interviewing facilitators, family carers and AS managers about their experiences. We assessed effectiveness on family carers' mental health using the Hospital Anxiety and Depression Scale (HADS) before and after receiving START (scores 0-42). We changed START's format by reflecting carer diversity more and increasing carer stories prominence, but core content or delivery processes were unchanged. All carers received START and attended every session. The mean fidelity score was 4.2. Mean HADS-total score reduced from baseline 18.4 (standard deviation 7.4) to follow-up 15.8 (9.7). Six (67%) carers scored as clinically depressed on baseline HADS and 2 (22%) at follow-up. Facilitators and carers rated START positively. Appropriately experienced third sector workers can be trained and supervised to deliver START and it remains effective. This has the potential for widened access at scale.