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Prescribed fire is an economical land management practice used to mitigate the global increase in woody plant encroachment (WPE) on rangelands. For woody plants that resprout following fire, the seedling stage may be vulnerable to fire. However, we know little about long-term growth of mesquite seedlings that survive fires. We measured seedling survival and post-fire gains in canopy cover of two cohorts (seeds planted in two different years) of honey mesquite (Prosopis glandulosa) seedlings in response to winter or summer fires in mixed-grass (~2,000 kg/ha) or tallgrass (~7,000 kg/ha) fuel types in the Southern Great Plains, USA. Post-fire seedling survival was highest to lowest in no fire, winter fire and summer fire, respectively, in both grass types and seedling cohorts. Canopy cover in all fire treatments and both cohorts increased to a greater level in mixed-grass than tallgrass over a 20-year period. Fire treatment had little effect on post-fire canopy cover gains. Rate of cover gain in mixed-grass (3.4 percentage units per year : PU/yr) was greater than a previously reported rate of 2.2 PU/yr for mature mesquite over 20 years at the same site. Lower mesquite seedling growth in tallgrass plots was likely due to a combination of greater pre-fire mortality, greater mortality from more intense fires, and greater post-fire grass competition for resources. Results suggest that mixed-grass, which is the dominant grass type in the region, may not be sufficiently competitive to slow the rate of mesquite seedling growth even after summer fire.