This study aims to elucidate the causal relationship between childhood maltreatment (CM) and subsequent mental health outcomes, including major depressive disorder (MDD), anxiety (ANX), post-traumatic stress disorder (PTSD), suicide attempts, and non-fatal self-harm. Utilising Mendelian Randomisation (MR) and genome-wide association studies (GWAS) data from individuals of European descent, this research applies a rigorous analytical methodology to large-scale datasets, overcoming the confounding variables inherent in previous observational studies. Genetic data were obtained from publicly available GWAS on individuals of European ancestry, focusing on Childhood Maltreatment (CM), Major Depressive Disorder (MDD), Anxiety (ANX), Post-Traumatic Stress Disorder (PTSD), Age at First Episode of Depression, Number of Depression Episodes, Non-fatal self-harm, and Suicide Attempts. Mendelian Randomisation (MR) analyses were conducted to investigate the causal impact of CM on these outcomes. Sensitivity analyses included IVW, MR Egger, WM, and MR-PRESSO. FDR corrections were applied to account for multiple testing. Results were presented as odds ratios (ORs) with confidence intervals (CIs). Significant associations were identified between CM and the likelihood of developing MDD (IVW: = 2.28, 95% = 1.66-3.14, < .001), ANX (IVW: = 1.01, 95% = 1.00-1.02, =.032), and PTSD (IVW: = 2.29, 95% = 1.43-3.67, =.001). CM was also linked to increased non-fatal self-harm (IVW: = 1.06, 95% = 1.04-1.08, <.001), higher frequency of depressive episodes (IVW: =0.31, 95% = 0.17-0.46, <.001), and earlier onset of depression (IVW: =-0.17, 95% = -0.32 to - 0.02, =.033). No significant association was found between CM and suicide attempts (IVW: = 1.09, 95% = 0.81-1.45, =.573). This study provides robust evidence that CM is a significant causal factor for MDD, ANX, PTSD, and non-fatal self-harming behaviours. It is associated with a higher frequency of depressive episodes and earlier onset of depression. These findings highlight the need for early intervention and targeted prevention strategies to address the long-lasting psychological impacts of CM.