Herein, selenium and nitrogen co-doped carbon quantum dots (Se/N-CQDs) were hydrothermally synthesized by using citric acid, histidine, and sodium selenite, which had sp3 and sp2 hybridized carbon atoms and showed excitation-dependent fluorescence behavior. Furthermore, due to the redox reaction of ABTS•+ and Se/N-CQDs, Se/N-CQDs had the excellent antioxidant capacity that it was demonstrated by scavenging ABTS•+ with the fading of blue. Based on the synergistic effect of Se/N-CQDs and Mn(II) on ABTS•+, Se/N-CQDs and ABTS•+, as a stable, sensitive, selective, and reproducible colorimetric sensor, was applied to the detection of Mn(II) with a detection limit of 1.69 μM and a linear range of 0 to 142.90 μM. More importantly, the probe was successfully applied to detecting Mn(II) in tap water, illustrating that it could be a promising tool for Mn(II) detection in water environments.