Magnesium (Mg) alloys are promising biodegradable materials but challenges remain due to their rapid degradation, especially in the potential use of Mg alloys as vascular stents. Surface modification techniques are the most straightforward way to address both the desired biocompatibility and inhibit the corrosion of Mg alloys. In this work, inspired by the functional moieties (catechols) of mussel adhesive proteins, a mimetic approach to construct organic protective conversion coatings on magnesium-zinc-manganese (MgZnMn) alloys is investigated. Based on the cross-linking of CA (catechol) and PEI (polyethyleneimine), a CA/PEI conversion coating is developed on a MgZnMn alloy. The CA/PEI conversion coating showed enhanced corrosion resistance due to the strong binding and aggregation of units. Moreover, such coatings could also provide enough primary amine groups, catechols and quinones, which can be used to immobilize further molecules. Heparin was further grafted onto the CA/PEI, endowing the conversion coating with the desired functionality. Vascular cell behavior on such a coating is also studied. The improved hemocompatibility, favorable anti-inflammatory ability, suppressed smooth muscle cell proliferation and enhanced endothelialization indicate the potential of such organic conversion coatings on MgZnMn alloys as vascular implants.