Motivated by the need to reduce toxic organic dyes in wastewaters, we studied the preparation of different amounts of Mn-doped cesium lead halide perovskite in PMMA matrix (CsPb x Mn1-x Cl y Br3-y @PMMA) composite fibrous membranes (CFM) as a photocatalyst to degrade dyes via a one-step in situ electrospinning method. The composite fibrous membranes exhibit excellent environmental stability, attributed to the protective three-dimensional network structure of the electrospun fibers, which shields the internal perovskite nanocrystals from direct contact with water and light. Structural characterization by X-ray diffraction (XRD) reveals that Mn2+ substitution induces a phase transformation from cubic CsPbBr3 to tetragonal CsPbCl3. The optimal Mn/Pb ratio of 2.5:1 achieves excellent photocatalytic activity, degrading 94.7% of methyl orange (MO) within 90 min under Xe lamp irradiation with a rate constant of 0.031 min-1, 4.6 times higher than that of the undoped sample. In addition, a single monochromatic CFM with a blue LED light source can be combined to construct a white LED (WLED) with good chromaticity coordinates (0.3267, 0.3298) and color temperature of 5775 K. This work highlights the great potential of Mn-doped perovskite composite fibrous membranes for practical photocatalytic and LED applications.