We study the formation of primordial black holes (PBHs) in the early Universe during a period of slow reheating after inflation. We demonstrate how the PBH formation mechanism may change even before the end of the matter-dominated phase and calculate the expected PBH mass function. We find that there is a threshold for the variance of the density contrast, σ_c ≃ 0.05, below which the transition occurs even before reheating, with this having important consequences for the PBH mass function. We also show that there is a maximum cutoff for the PBH mass at around 100 M⊙, below which the subdominant radiation bath affects PBH production, making the scenario particularly interesting for the recent LIGO observations of black hole mergers.