Neck pain is a burden to employers and employees amenable to improvement with neck/shoulder strengthening exercises. However, the benefits of such interventions on office workers' work ability remains unknown. This study evaluated the effects of a 12-week combined ergonomics and neck/shoulder strengthening exercise intervention (EET, n = 177, mean age 41.7 years, 26% female), versus a 12-week combined ergonomics and health promotion intervention (EHP, n = 173, mean age 43 years, 29% female) on work ability among office workers. Work ability was measured by a single question. Differences in the work ability score were analyzed using the intention-to-treat (ITT) and per-protocol (i.e., adherence ≥70%) analyses for between- and within-group differences at baseline, 12 weeks, and 12 months. A sub-group analysis was performed for neck cases, defined as reporting neck pain as ≥3 (out of 10). No significant between-group differences for work ability were observed in the general population, and subgroup of neck cases. A significant group-by-time interaction effect at 12 weeks and the trend for significance at 12 months favored the EET group in the per-protocol analysis of the neck cases. EET was effective in increasing work ability post-intervention and potentially, in the long-term, in symptomatic participants with ≥70% adherence to the intervention. However, EET was not superior to EHP.