A fiber-laser-based femtosecond dispersive wave laser source, operated within the 1.2-to 1.3-mu m bio-penetration wavelength window, and its application on nonlinear light microscopy were both demonstrated in this paper. This portable 1.2-mu m femtosecond fiber-optic source was composed of an all-fiber 1.55-mu m mode-locked erbium-doped fiber laser as the excitation source and a nonlinear fiber as a frequency up converter. The 1.2-mu m femtosecond radiations with an up to a 55-mW output power and a 125-fs pulsewidth were experimentally demonstrated. By the fiber-based 1.2-mu m femtosecond dispersive wave source, nonlinear laser scanning microscopy, including two-photon fluorescence and second-harmonic generation microscopy, was also performed within the 1.2-to 1.3-mu m bio-penetration window in this report. The fiber-based 1.2-mu m femtosecond light source, with a simple system configuration, turn-key operation, and a miniaturized package, shows great potential for nonlinear light microscopy and other related applications outside of the laboratory.