Understanding　the　dynamical　behavior　of　a　qubit　in　a　reservoir　is　critical　to　applications　in　quantum　technological　protocols,　ranging　from　quantum　computation　to　quantum　metrology.　The　effect　of　the　reservoir　depends　on　the　reservoir＇s　spectral　structure　as　well　as　on　the　qubit-reservoir　coupling　strength.　We　here　propose　a　measure　for　quantifying　the　non-Markovian　effect　of　a　reservoir　with　a　Lorentzian　spectrum,　based　on　the　maximum　qubit-reservoir　quantum　entanglement　that　can　be　extracted.　Numerical　simulation　shows　that　this　entanglement　exhibits　a　monotonous　behavior　in　response　to　the　variation　of　the　coupling　strength.　We　confirm　the　validity　of　this　measure　with　an　experiment　where　a　superconducting　qubit　is　controllably　coupled　to　a　lossy　resonator,　which　acts　as　a　reservoir　for　the　qubit.　The　experimental　results　illustrate　that　the　maximal　extractable　entanglement　is　progressively　increased　with　the　strengthening　of　non-Markovianity.