Remediation of the dye-contaminated water has received a broad interest due to the health problems and defects in the ecological system originating from the presence of such toxic materials in water. Herein, MnO@C nanocomposites as efficient adsorbent/photocatalyst materials for the removal of cationic and anionic dyes were synthesized via a simple and new precursor-assisted method. Manganese malonate was prepared and calcined at 350 °C for 1 h under air, argon, and H₂ atmospheres. Pristine MnO was obtained under argon and H₂. The obtained nanocomposites are highly crystalline and have sphere-like shapes as evidenced by TEM, HRTEM, and SAED. EDX analysis demonstrated the presence of C in the prepared samples. The study of the surface texture via the BJH method and V_a-t plots has revealed the presence of both microporous and mesoporous pores. BET surface areas of 22.7 and 50.1 m²/g were found for MnO@C nanocomposites prepared under Ar and H₂, respectively. Removal of MB as a cationic dye from water was explored via adsorption of the dye over the surface of MnO@C nanocomposites. The MnO@C sample prepared under H₂ exhibited the largest removal efficacy, removal of 97% of the dye occurs in 6 h. Furthermore, the MnO@C nanocomposite was utilized as a photocatalyst for removal of the anionic dye, eosin Y. Complete removal of eosin Y color was achieved after exposure to sunlight for 2 h. The mechanism of the photocatalytic degradation of eosin Y was investigated. The possible degradation products were detected through HPLC-UV analysis. The prepared nanocomposite showed good recyclability and even preserving its structure after 4 cycles.