Defect engineering has been widely explored as an effective route to modulate the electromechanical response of piezoelectric ceramics. However, achieving a high strain gain per defect design is often constrained by the competition between bias enhancement and defect-induced pinning. Here, we systematically investigate the defect-mediated electromechanical behavior of Mn-doped BiFeO₃–BaTiO₃-based ceramics with controlled defect concentrations. It is demonstrated that introducing an appropriate level of B-site aliovalent Mn dopants effectively amplifies the internal bias field while preserving ferroelectric switch ability leading to a pronounced bias-assisted strain amplification. In the optimal composition (x = 0.005), the strain increases from 0.02% to 0.23% at 3 kV/mm and from 0.06% to 0.36% at 4 kV/mm, corresponding to ~1050% and ~500% enhancements, respectively and a high large-signal piezoelectric coefficient d₃₃^* ≈ 900 pm/V. Structural and electrical analyses reveal that low Mn doping promotes the formation of dense nanodomains and facilitates the aging-induced ordering of defect dipoles, whereas excessive Mn incorporation induces strong lattice disorder and defect pinning, suppressing bias-field formation and strain response. These findings establish an effective defect–structure–bias-field design principle for enhancing electromechanical strain and strain-amplification efficiency in lead-free ferroelectric ceramics.