The present work demonstrates a facile hydrothermal approach to synthesize lanthanide‐doped carbon quantum dots (CQDs) with europium and/or gadolinium elements. Taking the advantage of broadband adsorption in the ultraviolet‐visible region, the doped QDs are directly used as building blocks for photo‐electrochemical (PEC)‐type photodetectors (PDs) and their performance is systematically investigated under various conditions. The europium (Eu) and gadolinium (Gd) co‐doped (C:EuGd) QDs exhibit better photo‐response than the single‐elemental doped ones and also show outstanding long‐term stability. According to the apparent response to light from 350 to 400 nm, the C:EuGd QDs are demonstrated to hold great potential for narrow‐band PDs. This work highlights the practical applications of lanthanide‐doped CQDs for PDs, and the results are beneficial for the development of elemental‐doped CQDs in general.