Low-dimensional systems host exotic quantum phenomena arising from dimensional confinement and strong electron correlations. Here, we report a prototypical case of site-specific mixed dimensionality in $\mathrm{C}{\mathrm{e}}_5\mathrm{Ag}{\mathrm{Pb}}_3$, which originates from inequivalent Ce1($6g$) and Ce2($4d$) sites. The frustrated quasi-two-dimensional Ce1 sublattice develops an incommensurate noncollinear antiferromagnetic order with moment modulation characterized by a propagation vector of $k_{\mathrm{z}}\approx 0.84$ below $T_{\mathrm{N}}=10\mathrm{K}$, exhibiting two-dimensional Ising critical behavior $\left[\beta =0.123\left(6\right)\right]$. The quasi-one-dimensional Ce2 chains remain magnetically disordered but host dynamic spin fluctuations decoupled from long-range magnetic order, revealed by a persistent $-H^2$-dependent component in negative magnetoresistance. The coexistence of long-range order and mixed-dimensional fluctuations drives the formation of a strongly correlated electronic state with a large γ of $345.6\mathrm{mJ}\mathrm{mo}{\mathrm{l}}^{-1}{\mathrm{K}}^{-2}$. Our work thus provides a unique, site-specific paradigm for disentangling the complex magnetism in bulk materials with mixed-dimensional motifs.

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• Magnetic order
• Magnetotransport
• Antiferromagnets
• Single crystal materials
• Crystal growth
• Neutron diffraction