The presence of antisite disorder in double perovskites manifests various intriguing properties like the spin-glass state, exchange bias, and memory effect. Here, we report the synthesis of a La1.5Sm0.5NiMnO6 compound that crystallizes in a monoclinic (P21/n) structure. The presence of multiple oxidation states of Ni(Mn) cations induces competing (ferromagnetic and antiferromagnetic) exchange interactions that originate an inhomogeneous spin state, as evident from observed magnetic anomalies in temperature-dependent magnetization measurements. A spin-glass (SG) state is evolved that manifests field cooling (HCF = 500 Oe) induced exchange bias (HEB∼153 Oe) below spin-glass temperature TSG (65 ± 1 K). The strength of the exchange bias is reduced after successive magnetization reversal cycles performed at 5 K. The reported magnetic training effect is explained within the frameworks of metastable magnetic disorder across frozen antiphase boundaries in the frustrated SG state. Measurements of frequency-dependent ac-susceptibility χ(ω) suggest critical slowing dynamics and memory effect in the proximity of TSG, which is described using a critical slowing model resulting in relaxation exponent zν = 1.99 ± 0.04 and τ0 = 8.91 x 10-7s. Employing first-principles calculations, we find the insulating ferromagnetic ground state of La1.5Sm0.5NiMnO6 in the ordered phase where Ni(Mn) appears to be in the 2+(4+) state. Further, the presence of antisite disorder eventually results in lower magnetic moments per formula unit, which is well corroborated by experimental observations. Our findings provide a pathway for designing host materials with inhomogeneous spin-frustrated systems and variable electronic states.