Background Clinical practice guidelines are not easily implemented, leading to a gap between research synthesis and their use in routine care. Aims To summarise the evidence relating to the impact of guideline implementation on provider performance and patient outcomes in mental healthcare settings, and to explore the performance of different strategies for guideline implementation. Method A systematic review of randomised controlled trials, controlled clinical trials and before-and-after studies comparing guideline implementation strategies v. usual care, and different guideline implementation strategies, in patients with severe mental illness. Results In total, 19 studies met our inclusion criteria. The studies did not show a consistent positive effect of guideline implementation on provider performance, but a more consistent small to modest positive effect on patient outcomes. Conclusions Guideline implementation does not seem to have an impact on provider performance, nonetheless it may influence patient outcomes positively.