In The Netherlands, on average 40% of all suicides concern patients treated by mental healthcare institutions (MHIs). Recent evidence indicates that implemented guideline recommendations significantly reduce the odds for patients to die by suicide. Implementation of the multidisciplinary guideline for diagnosis and treatment of suicidal behaviors is a main objective of the Dutch National Suicide Prevention Strategy. To this end, 24 MHIs that collectively reported 73% of patient suicides in 2015 received an educational outreach intervention offered by the national center of expertise. Aim: To investigate changes in levels of implementation of guideline recommendations; and to assess the degree of variation on suicide prevention policies and practices between MHIs. Methods: Implementation study with a prospective cohort design studying change over time on all domains of a Suicide Prevention Monitor, a guideline-based instrument assessing suicide prevention policies and practices within MHIs. Data were collected in six-month intervals between 2015 and 2017. Results: MHIs improved significantly on four out of ten domains: the development of an organizational suicide prevention policy; monitoring and trend-analysis of suicides numbers; evaluations after suicide; and clinician training. No improvement was measured on the domains pertaining to multi-annual training policies; collaborative care with external partners; recording and evaluation of suicide attempts; routine assessment of suicidality in all patients; safety planning and involving next of kin and carers. Furthermore, marked practice variation between MHIs was found which did not decrease over time. Conclusion: This study shows significant improvement in the implementation of four out of ten guideline-based suicide prevention policies in 24 specialist mental healthcare institutions in The Netherlands. The implementation level of suicide prevention policies and practices still appears to vary significantly between MHIs in The Netherlands.