We report herein a mild stereo‐ and regioselective dearomatization of quinolines using the simple low valent HCo(N2)(PPh3)3 complex that exhibits labile ligands. Conditions to form selectively, at room temperature, high‐valued 1,4‐bis‐borylated tetrahydroquinolines from simple starting heteroaromatic compounds have been developed. The efficient and selective functionalization of a large scope of quinolines bearing various electron‐donating or electron‐withdrawing substituents is presented, as well as the post‐modification of the resulting C–B bond. NMR and labelling studies are consistent with a cascade mechanism pathway, starting from an in situ generated paramagnetic bis‐quinoline cobalt(I) hydride complex. A first quinoline dearomatization followed by a cobalt(I)‐catalyzed Markovnikov hydroboration of the remaining double bond allows the introduction of the boronic ester group only at C4 position. DFT calculations particularly highlight the importance of the cobalt triplet state throughout the reaction pathway, and bring some rationalization for the observed C4 selective borylation.