Background Behavioural and cognitive interventions remain a credible approach in preventing loneliness and depression. There was a need to rapidly generate and assimilate trial-based data during COVID-19. Objectives We undertook a COVID-19 parallel pilot RCT of behavioural activation for depression and loneliness [the BASIL-C19 trial ISRCTN94091479]. We also assimilate these data in a COVID-19 living systematic review [PROSPERO CRD42021298788]. Methods Primary care participants (>=65 years) with long-term conditions were computer randomised to Behavioural Activation (n=47) versus care-as-usual (n=49). The single blinded primary outcome was the PHQ-9. Secondary outcomes included loneliness (De Jong Gierveld Scale). Data from the BASIL-C19 trial were included in a random effects meta-analysis of depression and loneliness. Findings The 12 months adjusted mean difference for PHQ-9 was -0.70 (95% CI -2.61 to 1.20) and for loneliness was -0.39 (95% CI -1.43 to 0.65). Secondary 12-month trial outcomes suggested evidence of benefit for behavioural activation. The BASIL-C19 meta-analysis (13 trials) found short-term reductions in depression (standardised mean difference [SMD]=-0.31, 95%CI -0.51 to -0.11) and loneliness (SMD=-0.48, 95%CI -0.70 to -0.27). There were few long-term trials, but there was evidence of some benefit (loneliness SMD=-0.20, 95%CI -0.40 to -0.01; depression SMD=-0.20, 95%CI -0.47 to 0.07). Discussion We found a signal of effect in reducing loneliness and depression in the BASIL trial. Living meta-analysis provides strong evidence of short-term benefit for loneliness and depression. Clinical implications Scalable behavioural and cognitive approaches should be considered as population-level strategies for depression and loneliness on the basis of the living systematic review. Funding This study was funded by National Institute for Health and Care Research (NIHR) Programme Grants for Applied Research (PGfAR) RP-PG-0217-20006.