This paper presents a diagnostic investigation and conservation of a ceramic mosaic in the Blessed Virgin Mary Church in Mostorod, Egypt. The panel, which is in the cave of the Church, depicts the Holy family and takes the name “Cave mosaic panel." However, various deterioration factors and previous restoration works negatively affected the mosaic panel. The panel suffered from many deterioration aspects, especially buckling, soot, dirt, salts, and loss of some pieces. The study aims to identify the type and structure of the ceramic mosaics, determine and diagnose deterioration aspects, and carry out the required conservation works of the panel to protect it in the future. A diagnostic study was carried out using Scanning Electron Microscopy with Energy Dispersive X-ray, X-Ray Diffraction and Fourier-Transform Infrared Spectroscopy. The results showed the chemical composition and the type of colorant oxides of the ceramics. Cobalt oxide was used to get dark blue glaze, with the use of alkali and lead alkaline as flux agents in the glaze layer, and the degrees of burning ceramic pieces higher than 1250 °C. Halite was detected between mosaic pieces. The results confirmed the use of animal glue as an adhesive for fixing some mosaic pieces. The treatment and conservation were carried out using mechanical followed by chemical cleaning methods, the dissolved salts were extracted by a cotton poultice, and the buckling of mosaics was treated by injection using Primal AC33. The lost parts of the mosaics were completed. Consolidation and protection using pF4 nanocomposite were performed.