Layered compounds $R\mathrm{S}{\mathrm{b}}_2$ ($R$ = light rare earth), which first hosted the discovery of non-rivial topological fermions in nonmagnetic $\mathrm{La}\mathrm{S}{\mathrm{b}}_2$, offer an exceptional platform for probing the interplay between magnetism and topological electronic states. Through high-field magnetotransport measurements and first-principles calculations, we report colossal magnetoresistance up to $\sim 6\times {10}^3\%$ at 28 T and ultrahigh carrier mobility exceeding $1.5\times {10}^4\mathrm{c}{\mathrm{m}}^2{\mathrm{V}}^{-1}{\mathrm{s}}^{-1}$ at 2 K in magnetic $R\mathrm{S}{\mathrm{b}}_2$ with $R=\mathrm{Pr}$, Nd, and Sm. Analysis of Shubnikov–de Haas oscillations further reveals a nontrivial Berry phase, corroborating topologically protected quasiparticles. Our calculations confirm the existence of nearly fourfold degenerate Dirac-like flat bands that remain robust under applied magnetic field and spin-orbit coupling, providing the topological nature across this material family. These findings not only describe a compelling magnetic topological semimetal system, but also provide quantitative insights into how magnetic ordering interacts with and sustains nontrivial topological states.

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• Magnetotransport
• Topological materials
• Quantum oscillation techniques