The article describes the synthesis, structure and magnetic investigations of a series of metal-organic framework compounds formed with Mn⁺² and Ni⁺² ions. The structures, determined using the single crystal X-ray diffraction, indicated that the structures possess two- and three-dimensional structures with magnetically active dimers, tetramers, chains, two-dimensional layers connected by polycarboxylic acids. These compounds provide good examples for the investigations of magnetic behaviour. Magnetic studies have been carried out using SQUID magnetometer in the range of 2-300 K and the behaviour indicates a predominant anti-ferromagnetic interactions, which appears to differ based on the M-O-C-O-M and/or the M-O-M (M = metal ions) linkages. Thus, compounds with carboxylate (Mn- O-C-O-Mn) connected ones, [C₃N₂H₅][Mn(H₂O){C₆H₃(COO)₃\}], I, [{Mn(H₂O)₃}{C₁₂H₈O(COO)₂}]$\cdots$H₂O, II, [{Mn(H₂O)}{C₁₂H₈O(COO)₂}], III, show simple anti-ferromagnetic behaviour. The compounds with Mn-O/OH-Mn connected dimer and tetramer units in [NaMn{C₆H₃(COO)₃}], IV, [Mn₂($\mu_3$-OH) (H₂O)₂{C₆H₃(COO)₃}]$\cdot$2H₂O, V, show canted-antiferromagnetic and anti-ferromagnetic behaviour, respectively. The presence of infinite one-dimensional -Ni-OH-Ni- chains in the compound, [NiR_2R(HR_2RO)($\mu_3$-OH)₂(C₈H₅NO₄], VI, gives rise to ferromagnet-like behaviour at low temperatures. The compounds, [Mn₃{C₆H₃(COO)₃}$₂], VII and [{Mn(OH)}₂{C₁₂H₈O(COO)₂}], VIII, have two-dimensional infinite -Mn-O/OH-Mn- layers with triangular magnetic lattices, which resemble the Kagome and brucite-like layer. The magnetic studies indicated canted-antiferromagnetic behaviour in both the cases. Variable temperature EPR and theoretical magnetic modelling studies have been carried out on selected compounds to probe the nature of the magnetic species and their interactions with them.