Background Mobile or seasonal migrant workers are at increased risk for acquiring malaria infections and can be the primary source of malaria reintroduction into receptive areas. The aim of this formative assessment was to describe access to malaria prevention and control interventions among seasonal migrant or mobile workers in seven regional states of Ethiopia. Methods A cross-sectional formative assessment was conducted using a qualitative and quantitative mixed-method design, between October 2015 and October 2016. Quantitative data were collected from organizations that employ seasonal migrant workers and were analyzed using Microsoft Excel and ArcGIS 10.8 (Geo-spatial data). Qualitative data were collected using in-depth interview from 23 key informants (7 seasonal migrant workers, and 16 experts and managers of development projects who had hired seasonal migrant workers), which were recorded, transcribed, translated, coded, and thematically analyzed. Results There were 1,017,888 seasonal migrant workers employed in different developmental organizations including large-scale crop cultivating farms, sugar cane plantations, horticulture, road and house construction work, and gold mining and panning. Seasonal migrant workers' housing facilities were poorly structured and overcrowded (30 people living per 64 square meter room) limiting the use of indoor residual spraying (IRS), and forcing seasonal migrant workers not to use long lasting insecticidal treated nets (LLINs). Seasonal migrant workers are engaged in nighttime activities when employment includes watering farmlands, harvesting sesame, and transporting sugar cane from the field to factories. Despite such high-risk living conditions, access and utilization of preventive and curative services by the seasonal workers were limited. Informal migrant worker employment systems by development organizations and inadequate technical and financial support coupled with poor supply chain management limited the planning and delivery of malaria prevention and treatment strategies targeting seasonal migrant workers. Conclusions Seasonal migrant workers in seven regions of Ethiopia were at substantial risk of acquiring malaria. Existing malaria prevention, control and management interventions were inadequate. This will contribute to the resurgence of outbreaks of malaria in areas where transmission has been lowered. A coordinated action is needed among all stakeholders to identify the size of seasonal migrant workers and develop and implement a comprehensive strategy to address their healthcare needs.