Background: Emergency laparotomy (EL) is a high-risk operation performed on an evolving demographic in an ageing first world. Objective frailty assessment using a validated index has the potential to improve risk stratification. The NSQIP-based 11-Item modified frailty index (mFI) - and shortened 5-Item version have demonstrated strong correlation with early mortality following EL. Primary aims were to assess the correlation between frailty, long-term mortality and other post-operative outcomes for older EL patients. Secondary aims were to compare indices in terms of value and predictive ability. Methods: A prospective multi-center observational study of older EL patients was conducted across five hospitals in New Zealand between 2017-2022. Predictive logistic regression models - unadjusted and adjusted for age, gender and ethnicity - were created for both indices and each outcome. Spearman’s p was used to assess correlation between the two indices. Outcomes: Frailty was assessed in 861 participants. Both models demonstrated strong correlation with long-term mortality, major complications, admission to ICU, rehabilitation, and 30-day readmission. The 11-Item mFI model demonstrated a greater association with early mortality - and independent relationships with reoperation and increased length of stay. Of participants classified as frail by the 11- but not 5-Item mFI, the majority had cardiovascular variables. Spearman p was 0.6 (p <0.001). Conclusions: Frailty was strongly correlated with long-term mortality and other important outcomes. Modified frailty indices are appropriate for the EL setting. Future research should focus on the addition of cardiovascular variables to the 5-Item mFI, as this may strengthen its predictive power of early mortality.