We herein present the synthesis, crystal structure, and magnetic properties of a new heterometallic chain of Mn-III and Ni-II ions, [Mn-2(saltmen)(2)Ni(pao)(2)(py)(2)](ClO4)(2) (1) (saltmen(2-) = N,N'-(1,1,2,2-tetramethylethylene) bis(salicylideneiminate) and pao(-) = pyridine-2-aldoximate). The crystal structure of 1 was investigated by X-ray crystallographic analysis: compound 1 crystallized in monoclinic, space group C2/c (No. 15) with a = 21.140(3) Angstrom, b = 15.975(1) Angstrom, c = 18.6212(4) Angstrom, beta = 98.0586(4)degrees, V = 6226.5(7) Angstrom(3), and Z = 4. This compound consists of two fragments, the out-of-plane dinner [Mn-2(saltmen)(2)](2+) as a coordination acceptor building block and the neutral mononuclear unit [Ni(pao)(2)(py)(2)] as a coordination donor building block, forming an alternating chain having the repeating unit [-Mn-(O)(2)-Mn-ON-Ni-NO-](n). In the crystal structure, each chain is well separated with a minimum intermetallic distance between Mn and Ni ions of 10.39 Angstrom and with the absence of interchain pi overlaps between organic ligands. These features ensure a good magnetic isolation of the chains. The dc and ac magnetic measurements were performed on both the polycrystalline sample and the aligned single crystals of 1. Above 30 K, the magnetic susceptibility of this one-dimensional compound was successfully described in a mean field approximation as an assembly of trimers ((MnNiMn)-Ni-...-Mn-...) with a (NiMnIII)-Mn-II... antiferromagnetic interaction (J = -21 K) connected through a ferromagnetic (MnMnIII)-Mn-III... interaction (J). However, the mean field theory fails to describe the magnetic behavior below 30 K emphasizing the one-dimensional magnetic character of the title compound. Between 5 and 15 K, the susceptibility in the chain direction was fitted to a one-dimensional Ising model leading to the same value of J. Hysteresis loops are observed below 3.5 K, indicating a magnet-type behavior. In the same range of temperature, combined ac and dc measurements show a slow relaxation of the magnetization. This result indicates the presence of a metastable state without magnetic long-range order. This material is the first experimental design of a heterometallic chain with S-T = 3 magnetic units showing a "single-chain magnet" behavior predicted in 1963 by R. J. Glauber for an Ising one-dimensional system. This work opens new perspectives for one-dimensional systems to obtain high temperature metastable magnets by combining high spin magnetic units, strong interunit interactions, and uniaxial anisotropy.