A sustainability index (SI) is a benchmark for guiding the activities of municipal water and wastewater (W and WS) utilities toward sustainability. The SI has been used in Sweden since 2014 as an internal benchmark management tool facilitating the strategic work of organizations. This study assesses how the SI was applied in practice by eight utilities (Orgs 1–8) from 2015 to 2019. The study was performed using in-depth semi-structured interviews with managers of the utilities and analyzing 32 SI reports and other documents. The utilities generally perform better in all parameters compared with the national average. Three utilities had improved results during the study period, while the other five had unchanged development. Five utilities use the SI index to improve their performance, and three use the SI results to grade the organization. All utilities are concerned with sustainability and wish to improve their performance. The study found that the parameter “Status of W and WS fixed facilities” was the furthest from sustainability. The study also clarifies that the technical committee has the primary responsibility to ensure that the technical office meets the sustainability objectives.