Ultra-small (1.6 nm), water-soluble, white light-emitting (WLE), highly stable (~1 year)BSA templated metallic Mg nanoclusters (fluorescent magnesium nanoclusters=FMNCs) are developed using the green and facile route. Synthesis was facilitated by the reduction of magnesium salt, where, template bovine serum albumin is utilized as a reducing agent and ascorbic acid act as a capping agent to impart stability in water, thereby obtaining stabilized Mg0 nanocluster. In solution, stabilized Mg0 nanoclusters produce white light (450-620 nm with FWHM~120 nm) upon 366 nm light excitation. This white light emission was found to have a CIE coordinate of 0.30, 0.33 [pure white light CIE (0.33, 0.33)]. Taking, advantage of WLE and ultrasmall size, FMNCs were used for in-vivo fluorescence imaging of HaCaT cell lines, yielding blue (τ = 2.94 ns, with relative of QY= 1.2 % w.r.t QS), green (τ = 3.07 ns; relative quantum yield of 4.6% w.r.t R6G) and red (τ = 0.3 ns) images. Further, incubation of FMNCs with HEK293 (Human embryonic kidney cell) and cancerous MDA-MB-231 (Breast cancer cell line) human cell lines yielded 100 % cell viability. Current work is envisioned to contribute significantly in the area of science, engineering, and nanomedicine.