Multivariate lanthanide metal–organic frameworks (MOFs) provide a versatile platform for engineering materials with tunable multifunctional properties. Here we report a family of isostructural carborane-based MOFs of general formula {[(Ce_yEr_1−y)₃(mCB-L)₄(NO₃)(DMF)_x]_n·Solv}, comprising homometallic Ce and Er frameworks together with mixed analogues with various Ce : Er ratios. Element-selective X-ray absorption and magnetic circular dichroism (XAS-XMCD) on Ce₂Er analogue identifies Er³⁺ and mixed-valent cerium (Ce³⁺/Ce⁴⁺ ≈ 0.8 : 0.2), and show that the magnetization is accurately reproduced by a linear combination of the individual lanthanide contributions. Field-induced slow magnetic relaxation is observed in all compounds with moderate effective barriers (U_eff/k_B ≈ 5–24 K), while the series also exhibits magnetocaloric properties, maximized for the Er framework. Optically, the materials display multi-band emission combining ligand-centered violet (420 nm), Ce³⁺ yellow-green (560 nm), and Er³⁺ telecom-band near-infrared emission (1554 nm). Notably, the absence of Ce → Er energy transfer in mixed MOFs enables simultaneous dual visible-NIR output. These results establish Ce/Er carborane-based MOFs as compositionally programmable multifunctional platforms integrating slow magnetic relaxation, magnetocaloric response, and dual visible–NIR luminescence, with potential applications in magneto-photonic tagging, multiplexed optical readout, cryogenic cooling, and quantum technologies.