Mid-infrared supercontinuum (SC) generation from 2.6 to 4.1 μm is demonstrated in a single-mode erbium-doped fluoride glass fiber amplifier pumped at 976 nm and seeded by an optical parametric generation (OPG) source emitting 400 ps pulses at 2.75 μm. Up to 82% of the SC power is generated beyond 3 μm. This novel and simple in-amplifier SC generation approach is promising for the development of compact and efficient sources operating in the mid-infrared, especially in the 3-5 μm band.