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Extending the limit of charge-carrier mobility in semiconductors has been a long-standing pursuit in material science and its applications. Herein, we investigate the electron mobility via cyclotron resonance in undoped diamond under continuous-wave photoexcitation, whereby the density of charge carriers can be reduced to 10⁸cm⁻³ or 1/10 of the previous detection limit [K. Konishi et al., Appl. Phys. Lett. 117, 212102 (2020)]. For low-density electrons, which obviate the effects of carrier-carrier scattering as a broadening mechanism, we observe an extraordinarily narrow cyclotron resonance spectrum. After correcting for the microwave power broadening, the highest intrinsic mobility value of 100×10⁶ cm²V⁻¹s⁻¹ is obtained at 3 K, which is a 16-fold increase of the mobility compared with the previous record in diamond. Our result is beneficial for the design and application of diamond radiation detectors implemented for their practical use at cryogenic temperatures.