To examine the associations of age, lifestyle and work-related factors, and particularly work engagement with the work ability index (WAI) and its sub-dimensions. Step-wise regression analysis with a sample of Finnish firefighters (n = 403) was used. The outcome variables were the WAI and its six sub-dimensions. The independent variables consisted of age, lifestyle variables (alcohol consumption, BMI, smoking, physical exercise, and sleep problems), working conditions (job demands, physical workload, supervisory relations, and task resources), and work engagement. The outcome variables and all the variables related to lifestyle, working conditions, and work engagement were measured in 2009. Work ability at baseline 10 years earlier was adjusted for in the models. Work engagement, age, physical exercise, sleep problems, and physical workload were associated with the WAI. All independent variables, except BMI and alcohol consumption, were associated with at least one sub-dimension of the WAI after controlling the baseline WAI. Lifestyle variables, working conditions, and work engagement were more strongly related to the subjective WAI sub-dimensions than to the two more objective WAI sub-dimensions. Work engagement was significantly associated with work ability even after adjusting for various factors, indicating its importance in promoting work ability. Other key factors for good work ability were frequent exercise, good sleep, non-smoking, low job demands, low physical workload, and high task resources. More specifically, this study suggests that in maintaining work ability, it is valuable not only to promote lifestyle factors or working conditions, but also to enhance employees' positive state of work engagement.