This systematic review aimed to identify 1) the effect of mindfulness training on pre-post measures of anxiety and attention among adults experiencing high levels of generalised anxiety; and 2) the impact of predictors, mediators and moderators on post-intervention changes in anxiety or attention. Trait mindfulness and distress measures were included as secondary outcomes. A systematic search was conducted in November 2021 in electronic databases using relevant search terms. Eight articles comprising four independent studies were included (N= 334). All studies included participants diagnosed with generalised anxiety disorder (GAD) who participated in an 8-week manualised program. The meta-analysis indicated that mindfulness training had a large effect on anxiety symptoms (g= -1.92, 95%CI[-3.44, -0.40]) when compared to inactive (i.e., care as usual, waitlist) or non-specified (i.e., condition not defined) controls. However, a significant effect was not found when compared to active controls. Effects for depression, worry and trait mindfulness did not reach statistical significance, despite small-large effect sizes favouring mindfulness compared to inactive/non-specified controls. Our narrative review found evidence that changes in aspects of trait mindfulness mediate anxiety reduction following mindfulness training. However, a small number of studies were available for inclusion in the review, with high risk of bias and low certainty of evidence present. Overall, the findings support the use of mindfulness training programs for GAD and indicate mechanisms that may differ from those involved in other cognitive therapy approaches. Further RCTs with evidence-based controls are needed to clarify techniques most beneficial for generalised anxiety to support individually tailored treatment.Supplementary InformationThe online version contains supplementary material available at 10.1007/s12144-023-04695-x.