A novel molecule-based magnetic polymer Mn(N₃)₂(btr)₂1 (btr = 4,4‘-bi-1,2,4-triazole) was synthesized and characterized crystallographically and magnetically. 1 crystallizes in the monoclinic system, space group P2₁/c, formula C₈H₈N₁₈Mn, with a = 12.2831(4) Å, b = 6.3680(1) Å, c = 10.2245(3) Å, β = 105.064(1)°, and Z = 2. Bridged by end-to-end azides, the Mn²⁺ ions form a two-dimensional layer with (4,4) topology; the layers are further connected to the three-dimensional network by the weak hydrogen bonds between ligands of btr. Magnetic studies on a polycrystalline sample show the existence of strong antiferromagnetic couplings between the adjacent Mn²⁺ ions, and the Neél temperature is T_N = 23.7 K. In the ordered state below T_N, detailed investigations on an oriented single-crystal sample of 1 reveal that the hidden spin-canting, metamagnetic transition, and spin-flop transition can appear in different circumstances. The ground state is of an antiferromagnet with hidden spin-canting. An external field applied along the b direction parallel to the manganese azide layer can lead to a first-order metamagnetic phase transition, while a spin-flop transition may occur when the field is applied along the a* direction that is perpendicular to the manganese azide layer. Magnetic phase diagrams in both the T−H^a* and the T−H^b planes were determined. Possible spin configurations before and after the transitions were proposed. Analyses on the experimental data give the following intrinsic parameters:  the intra- and interlayer coupling J ≈ −3.5 cm^-1 and J^a* = 6 × 10^-4 cm^-1, the anisotropy field H_A = 0.2 kOe, the exchange field H_E = 387.8 kOe, and the anisotropy parameter α = 5 × 10^-4. The small J^a* and α show 1 to be a good example of a two-dimensional Heisenberg system.