Dopamine (DA), a key neurotransmitter biomarker, is closely associated with physiological states. Developing high-performance catalysts and the corresponding sensors for real-time in vivo DA monitoring remains significant. Recently, metallene, as an emerging two-dimensional catalyst, demonstrates potential in biomarker detection. However, large-scale synthesis and enhanced sensing activity remain critical challenges. Here, a single-atom lanthanum-loaded metallene catalyst (LaSA-Co MAs) was synthesized via a facile one-step ultrasonic reduction strategy, which demonstrates excellent sensing sensitivity for DA (0.17 μA·μM−1·cm−2) and a low limit of detection (23 nM) within the concentration range of 1–1000 μM, exhibiting a 34-fold enhancement compared to pristine Co MAs. This superior performance originates from the orbital hybridization and electron transfer between the LaSA and Co MAs, which facilitate the desorption/activation and reduce the activation energy for DA electrooxidation. Notably, this synthetic route is readily scalable through parameter optimization, achieving gram-scale production capacity (1.23 g per batch). Furthermore, an implantable flexible electrochemical sensor derived from the LaSA-Co MAs enables highly sensitive, reliable real-time monitoring of endogenous DA in live rat brains without inducing neuroinflammation. This work advances the design and application of the single atom-metallene hybrid for flexible electrochemical sensors.