This study analyses how social workers incorporate loneliness and social isolation into social assessment and diagnostic processes with older adults over sixty-five years. A mixed-methods design was used, involving a total of 425 participants (including social workers, older adults, and other stakeholders) through individual interviews (n = 30), focus groups (n = 44), and a questionnaire (n = 351), conducted in Barcelona (Spain). The findings show that a large majority of social workers (74.4 percent) report always or often including loneliness in the social diagnosis, indicating a high level of professional awareness. However, its translation into systematic diagnostic practices remains uneven and strongly influenced by organizational factors, workload, and institutional context. Detection is mainly based on interviews and observation rather than on standardized tools, and priority tends to be given to risk-related forms of loneliness over emotional or existential experiences. In addition, many older adults do not verbalize loneliness at early stages, reinforcing the need for longitudinal and relational diagnostic processes. The study highlights the importance of strengthening diagnostic frameworks, improving professional training, and creating organizational conditions that enable more coherent, preventive, and evidence-informed responses to loneliness in later life.