A new oxide, LaMnNiMnO, was prepared by high-pressure and high-temperature synthesis methods. The compound crystallizes in an AA'BB'O-type A-site and B-site ordered quadruple perovskite structure. The charge combination is confirmed to be LaMn Ni Mn O, where La and Mn are 1:3 ordered at the A and A' sites and the Ni and Mn are also distributed at the B and B' sites in an orderly fashion in a rocksalt-type manner, respectively. A G-type antiferromagnetic ordering originating from the A'-site Mn sublattice is found to occur at T ≈ 46 K. Subsequently, the spin coupling between the B-site Ni and B'-site Mn sublattices leads to an orthogonally ordered spin alignment with a net ferromagnetic component near T ≈ 34 K. First-principles calculations demonstrate that the A'-site Mn spins play a crucial role in determining the spin structure of the B and B' sites. This LaMnNiMnO provides a rare example that shows orthogonal spin ordering in the B and B' sites assisted by ordered A-site magnetic ions in perovskite systems.