Carbon nitrides synthesized by thermal polycondensation of melamine at 700 °C exhibit photoluminescence (PL) ranging from 400 to 650 nm. This broad PL is attributed to band to band transitions and bandtail transitions of lone pair (LP) states of intra-tri-s-triazine and inter-tri-s-triazine nitrogens. The proposed PL mechanism is further confirmed by diffusion reflectance spectroscopy, as well as time-resolved and temperature-dependent PL. This intense fluorescence is stable at different pH and resistant to UV exposure, suggesting that this inexpensive broadband luminescent material could be significant for white light-emitting (WLE) applications. Thus, quasi-WLE films and membranes with designed patterns are fabricated by embedding the carbon nitrides into polymethyl methacrylate. Moreover, even broader PL (400 to 740 nm) is acquired in composite films composed of carbon nitrides, further suggesting that the carbon nitrides are robust candidates for WLE.