Information Security Management (ISM) is a systematic initiative in managing the organisation's information security. ISM can also be defined as a strategic approach to addressing information security (IS) risks, breaches, and incidents that could threaten the confidentiality, integrity, and availability of information. Although organisations have complied with ISM requirements, security incidents are still afflicting numerous organisations. This issue shows that the current implementation of ISM is still ineffective. The ineffective ISM implementation illustrates the low maturity level. To achieve a higher level of maturity, organisations should always evaluate their ISM practices. Several maturity models have been developed by international organisations, consultants, and researchers to assist organisations in assessing their ISM practices. However, the current models do not evaluate ISM practices holistically. The measurement dimensions in current models are more focused on assessing certain factors only. This caused the maturity assessment to be not executed comprehensively. Therefore, this study aims to address this shortcoming by proposing a comprehensive maturity assessment model that takes into account ISM success factors to evaluate the effectiveness of the implementation. This study adopted a mixedmethod approach, which comprises qualitative and quantitative studies to strengthen the research finding. The qualitative study analyses the existing literature and conducts interviews with nine industry practitioners and six experts while the quantitative study involves a questionnaire survey. The data obtained from the qualitative study were analysed using content analysis while the quantitative data employed statistics analysis. The study identified fourteen success factors and fifty-seven maturity dimensions, which each contains five maturity levels. The proposed model was evaluated through experts' reviews to ensure its accuracy and suitability. The evaluation shows that the model can identify the ISM maturity level systematically and comprehensively. This model will ultimately help the organisations to improve the weaknesses in the implementations thus diminishing security incidents.