Loading margin sensitivity analyses are often performed for voltage stability assessment (VSA) and control due to the complexity of such applications. Traditional power system algebraic model based loading margin sensitivity analyses require full detailed system models and non-trivial amount of computation, therefore they are not good candidate for online applications. Based on the coupled single-port circuit concept, this paper proposes a new method of measurement-based wide-area loading margin sensitivity (WALMS) for VSA and control that is suitable for online applications. Enabled by wide-area measurement systems and their measurements, the proposed model derives the analytical expression of the sensitivity of loading margin (LM) versus active power injection, reactive power injection and generator terminal voltage. Consequently, the WALMS is computational more efficiently than its model-based counterparts. Based on the WALMS, a multi-step control strategy is developed to improve voltage stability. Simulation results on the IEEE 39-bus system verify the WALMS and validate the multi-step control strategy benchmarked with traditional continuation power flow when system is close to voltage collapsing point. Further tests on the IEEE 14, 57 and 300-bus systems are performed to demonstrate the applicability of the WALMS. Evidently, the multiple simulations results show that the measurement-based WALMS has great potential to be integrated to online VSA and control applications.