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Neutron diffraction and magnetic susceptibility studies of orthorhombic single crystal SrMnSb₂ and hole doped Sr_0.97K_0.03MnSb₂ confirm the three dimensional antiferromagnetic (AFM) ordering of the Mn²⁺ moments at T_N = 297(3) K for SrMnSb₂ and T_N = 305(3) K for the hole doped compound. Neutron scattering show strong quasi two-dimensional AFM correlations that persist to almost twice T_N. In conjuction with susceptibility measurements, this observation seems to rule out a recently suggested ferromagnetic phase above T_N in SrMnSb₂. We also report detailed analysis of de-Haas van-Alphen oscilations observed on the parent and doped compounds.