The causes of widespread mussel declines remain poorly understood, and few assessment methods quantify the extent of decline and overall assemblage health. These combined limitations pose challenges to policy and resource management efforts to protect this highly imperiled group before it’s too late. Most previous studies focused on specific streams, and none have critically evaluated potential causes of mussel declines across a large geographical area. Similarly, available assessment tools are specific to particular streams or regions and depend on untested assumptions or subjective assessments of species’ tolerance to various factors. Therefore, we developed a broadly applicable, objective mussel assemblage health index (MAHI). MAHI includes 4 metrics representing fundamental aspects of assemblage or population health (species loss, recruitment, abundance, and population growth). Metrics are scaled from 0 to 10 (representing increasing health), and the composite score is the unweighted mean of the 4 metrics. MAHI effectively discriminated healthy (median score 58.3) and degraded (3.2) assemblages. The composite score is robust to missing values. Composite scores were similar to independent scores provided by experts, but MAHI eliminates variation due to differences in individual perception or experience. MAHI can be applied to streams across eastern North America using commonly collected mussel survey data, and it may be adapted to other regions. MAHI is free of untested assumptions or subjective assessments, and it accounts for expected differences in mussel assemblages due to stream size, biogeographic region, and other intrinsic factors. Importantly, MAHI does not correlate to other commonly used assessments like fish and aquatic insect Index of Biotic Integrity (IBI) scores which further highlights the need for this assessment adoption to ensure we are capturing stressors to freshwater mussels to enable early detection of mussel declines, better understand the causes of declines, and evaluate the effectiveness of conservation strategies and management actions.