Visible and mid-infrared (MIR) lasers are crucial in a wide range of practical applications. To develop lasers with high beam quality, excellent wavelength tunability, and high output power in the visible and MIR regions, nonlinear frequency conversion of fiber lasers has emerged as a highly promising approach. Here, the operation principles and recent progress in the nonlinear frequency conversion of various types of fiber lasers toward visible and MIR ranges are overviewed.