Invasive watermilfoil strains can differ in their growth, spread, impacts, and herbicide response. For example, strains of Eurasian (Myriophyllum spicatum L.) and hybrid (M. spicatum × M. sibiricum) watermilfoil have been characterized as resistant or susceptible to specific herbicides (e.g., fluridone and 2,4-D). Identifying resistant and susceptible strains can inform managers as to whether a specific herbicide should be used to treat a water body. However, to date, no centralized location existed to house and share M. spicatum and M. spicatum × M. sibiricum strain and herbicide response information. To address this need, we built MilfoilMapper, a publicly available, user-friendly, R Shiny application that houses invasive watermilfoil strain distribution and herbicide response information. To date, we have identified 290 strains from over 300 lakes across the United States sent by state agencies, aquatic plant managers, and citizen scientists. Although some strains are found only in a single lake, some strains have been found in multiple lakes. Therefore, strain information obtained from either the field or the lab can be applied to additional lakes where these strains are found. We encourage people to incorporate genetic surveying and monitoring into their M. spicatum management plans to help identify strains that should be prioritized for herbicide characterization. We believe MilfoilMapper will facilitate and encourage these actions by providing a centralized, interactive platform for tracking M. spicatum and M. spicatum × M. sibiricum strain data enabling lake managers, stakeholders, and state agencies to share experiences and resources to improve the efficacy and efficiency of invasive M. spicatum management.