Background: Providing care to an older family member can negatively impact the psychological health and wellbeing of the carer. Interventions that cultivate self-compassion have demonstrated efficacy in supporting psychological health and wellbeing among various populations. However, there is limited research into self-compassion with family carers of older adults, and there are no interventions that target self-compassion for this group specifically. Aim and Objectives: This PhD study aimed to plan and design a self-compassion intervention for family carers of older adults. It had three objectives: identify and review evidence to determine current knowledge about family carers of older adults and self-compassion; identify and develop an evidence-based theory of the likely or anticipated process of change in a self-compassion intervention for family carers of older adults; and identify the key design objectives and key features of the self-compassion intervention for use with family carers of older adults. Design and Assumptions: The PhD study was underpinned by pragmatism and used evidence-based, theory-based, person-based, and co-design approaches to intervention development. It had a sequential mixed model design that was structured into three stages: Stage I – reviewing the evidence-base; Stage II – identifying and developing theory; and Stage III – qualitative research and co-design. Stage I Methods and Findings: Stage I encompassed two reviews: an integrative review of self-compassion and health outcomes for family carers of older adults; and a scoping review of mindfulness- and compassion-based interventions with family carers of older adults. The reviews confirmed compassion and mindfulness for family carers of older adults as developing areas of interest. For self-compassion in particular, the reviews also found that studies conducted to date were limited in number, scope, and quality. Stage II Methods and Findings: Stage II involved a conceptual analysis and a cross-sectional survey. The conceptual analysis outlined a theoretical rationale for self-compassion as an intervention target for family carers of older adults and explored the potential mechanism of change. This rationale was tested and confirmed in an international cross-sectional survey of family carers of older adults. The survey included self-report measures of compassion for self, to others, from others; mindfulness; depression, anxiety, stress; coping strategies; and emotion regulation difficulties; and eight free-text comment boxes. Confirmatory factor analyses (n = 171) established the validity of the Compassionate Engagement and Action Scales with the study population. Path analysis (n = 139) found increased self-compassion and dispositional mindfulness was significantly associated with lower psychological distress (R2 = .65), and this was mediated by reduced emotion regulation difficulties (R2 = .52). Qualitative content analysis (n = 105 participants, 245 comments) identified that: family carers perceived a lack of compassion for themselves and from others; there were several barriers to carers’ openness to receiving compassion; and advanced needs of the care recipient negatively influenced carers’ compassion. Stage III Methods and Findings: Stage III included qualitative interviews and co-design activities. Interviews exploring perceptions and contextual issues relevant to the intervention’s development were conducted with 14 family carers of people living with dementia (Australia) and 14 professional stakeholders (Australia and UK). Reflexive thematic analysis identified that the intervention should target family carers of people living with dementia specifically; be situated within the concept of compassion more broadly; address misperceptions, fears, blocks, and resistances to self-compassion; and target feelings of shame, guilt, and self-criticism. Intervention guiding principles, psychological theory, and a co-design group (n = 6 Australian-based family carers of people living with dementia) further informed the intervention’s design. Collectively, this process determined that the needs of family carers of people living with dementia were best met by tailoring an existing intervention: group-based Compassion-Focused Therapy. Conclusions: Using a systematic and comprehensive approach to planning and designing an intervention, this PhD study established the current state of knowledge within the field; proposed, tested, and confirmed a conceptual model about the role of compassion and its mechanism of change in the psychological health of family carers of older adults; and validated a measure of compassion with the study population. Based on the needs, preferences, and co-design input of intended intervention users, the PhD study also defined the key design objectives and key features of the self-compassion intervention. Overall, group-based Compassion-Focused Therapy for family carers of people living with dementia was identified as an optimum intervention.