The Massively Multiplexed Spectrograph (mxSPEC) is a new instrument concept that takes advantage of modern high-speed large-format focal plane arrays (FPAs) and high efficiency bandpass isolation filters to multiplex spectra from many slices of the telescope field simultaneously onto the FPAs within a single grating spectrograph. This design greatly reduces the time required to scan a large telescope field, and with current technologies can achieve more than a factor of 50 or more improvement of the system efficiency over a conventional long-slit spectrograph. Furthermore, several spectral lines can be observed at the same time with proper selection of the diffraction grating, further improving the efficiency of this design to more than two orders of magnitude over conventional single-slit, single-wavelength instrument. This paper describes an experimental, proof-of-concept, 40-slit full-disk spectrograph that demonstrates the feasibility of this new instrument concept and its potential for solar physics research including helioseismology, dynamic solar events, and global scale magnetic field observation of the solar disk and the corona. We also present the preliminary design of a 4-line, 55-slit spectroheliograph that can serve as the template for the instruments of the next generation synoptic solar observatory.