Ni‐mediated trifluoromethylation of an aryl−Br bond in model macrocyclic ligands (Ln−Br) has been thoroughly studied, starting with an oxidative addition at Ni0 to obtain well‐defined aryl‐NiII‐Br complexes ([Ln−NiII]Br). Abstraction of the halide with AgX (X=OTf− or ClO4−) thereafter provides [Ln−NiII](OTf). The nitrate analogue has been obtained through a direct C−H activation of an aryl−H bond using NiII salts, and this route has been studied by X‐ray absorption spectroscopy (XAS). Crystallographic XRD and XAS characterization has shown a tight macrocyclic coordination in the aryl−NiII complex, which may hamper direct reaction with nucleophiles. On the contrary, enhanced reactivity is observed with oxidants, and the reaction of [Ln−NiII](OTf) with CF3+ sources afforded Ln−CF3 products in quantitative yield. A combined experimental and theoretical mechanistic study provides new insights into the operative mechanism for this transformation. Computational analysis indicates the occurrence of an initial single electron transfer (SET) to 5‐(trifluoromethyl)dibenzothiophenium triflate (TDTT), producing a transient L1−NiIII/CF3. adduct, which rapidly recombines to form a [L1‐NiIV‐CF3](X)2 intermediate species. A final facile reductive elimination affords L1−CF3. The well‐defined square‐planar model system studied here permits to gain fundamental knowledge on the rich redox chemistry of nickel, which is sought to facilitate the development of new Ni‐based trifluoromethylation methodologies.