To our knowledge, no research has reported long-term follow-up results from studies comparing collaborative care to consultation liaison in general practice. We have earlier reported 6-month follow-up. In this article, we report the 18-month follow-up results from the two Danish Collabri Flex studies. We compared collaborative care to consultation liaison in two randomized controlled trials for persons with anxiety disorders and depression, respectively. Both interventions sought to improve the quality of depression and anxiety care, but they differed in the extent mental health specialists were involved. As part of the consultation liaison intervention, psychiatrists and care managers could provide advice and suggestions to the general practitioner (GP). In the collaborative care intervention, psychiatrists and care managers could provide advice and suggestions, and care managers could help the GP manage patient care. 18 months after randomization, we collected follow-up data. Outcomes included anxiety symptoms (BAI), depression symptoms (BDI-II), well-being (WHO-5), functional disability (Sheehan Disability Scale), general psychological symptoms (SCL-90-R), and others. In the depression trial, we found a statistically significant difference in depression symptoms between intervention groups at 18-month follow-up, in favor of collaborative care (4.4, 95%CI 2.8-7.0, ≤ 0.001). Many other outcomes showed significant differences between groups, such as anxiety symptoms, functional level, well-being, general psychological symptoms, and self-efficacy. In the anxiety trial, we found no statistically significant difference between groups in anxiety symptoms (1.2, 95%CI -0.3-2.7, ≤ 0.126). In this trial, significant differences between groups were found in outcomes measuring depression symptoms and general psychological symptoms but not in outcomes measuring functional level, well-being or self-efficacy. In both trials, no differences were found between groups on employment/education or sick leave measures. However, the collaborative care group in both trials had fewer contacts with private practicing psychologists during the 18 months follow-up. At 18-month follow-up, we found a statistically significant difference between allocation groups, when measuring depression symptoms in the depression trial but not when measuring anxiety symptoms in the anxiety trial. Further, the collaborative care intervention may have the potential to reduce referrals to private practicing psychologists, a service that is in high demand in Denmark. ClinicalTrials.gov identifier: NCT03113175 and NCT03113201.