This research aimed to assess the economic, allocative, and technical efficiency levels and their determinants among small-scale onion farmers in the Mecha and Fogra districts of Amhara region, Ethiopia. The data was collected from a total of 374 onion farmers. The study employed a stochastic frontier model to estimate the technical, allocative, and economic efficiency of smallholder onion producers and a Two-Limit Tobit model to identify the causes of inefficiencies. The specification test result shows that the Cobb-Douglas (CD) production function best fits the data. The study revealed that land, labor, urea, NPS, and insecticide and herbicide significantly and positively influenced onion production, as expected. The technical, allocative, and economic efficiencies of the sampled onion farmers were estimated to be 69.2%, 75.6%, and 52.6%, respectively. The results further show that the age, livestock ownership, family size, technology (pump) adoption, improved seed, extension services on seed, extension service on harvest, and total income of the household were associated with higher levels of onion production efficiency in the study area. With the current inputs and technology, onion producers in the study area can potentially increase their output by 30.8% and/or lower their average cost of production by 24.4%, given the current level of production. Therefore, government policies and initiatives should support onion growers by improving access to agricultural inputs (seed and fertilizer) and quality of extension services.