The novel LnIII {Ln = Dy (1-Dy), Gd (2-Gd), and La (3-La)} benzimidazolium tricarboxylate coordination polymers have been synthesized, and their magnetic properties have been investigated. Single-crystal X-ray analysis revealed that 1-Dy, 2-Gd, and 3-La are one-dimensional coordination polymers with general molecular formulas of {[Ln(L)2(H2O)4]·(6Br)}∞ [L = 3,3′,3′′-((2,4,6-trimethylbenzene-1,3,5-triyl)tris(methylene))tris(1-(carboxymethyl)-benzimidazolium)], which crystallized in the monoclinic, P21/c space group. The solid-state packing of these coordination polymers shows the spiral propagation in a one-dimensional direction. The direct current (dc) magnetic data (susceptibility and magnetization) were collected for 1-Dy and 2-Gd. The alternating current (ac) magnetic measurement for 1-Dy at zero field shows the characteristic signature of single-molecule magnets (SMMs) at low temperatures but without clear maxima. Further, to rationalize the experimentally observed magnetic behavior and to understand the factors affecting the dynamic magnetic behavior of 1-Dy, we performed detailed completed active space self-consistent field (CASSCF) based calculations on 1-Dy. Our detailed theoretical analysis suggests that the hydrogen bonding interaction between the coordinated water molecule and the Br- counteranion increases the equatorial electron density, eradicating the slow relaxation in 1-Dy. © 2022 American Chemical Society.