The nature of the low-frequency 1/f noise in electronic materials and devices is one of the oldest unsolved physical problems (f is the frequency). The fundamental question of the noise source-fluctuations in the mobility vs. number of charge carriers-is still debated. While there are several pieces of evidence to prove that the 1/f noise in semiconductors is due to the fluctuations in the number of the charge carriers, there is no direct evidence of the mobility fluctuations as the source of 1/f noise in any material. Herein, we measured noise in an h-BN encapsulated graphene transistor under the conditions of geometrical magnetoresistance to directly assess the mechanism of low-frequency electronic current fluctuations. It was found that the relative noise spectral density of the graphene resistance fluctuations depends non-monotonically on the magnetic field (B) with a minimum at approximately μ0B ≅ 1 (μ0 is the electron mobility). This observation proves unambiguously that mobility fluctuations are the dominant mechanism of electronic noise in high-quality graphene. Our results are important for all proposed applications of graphene in electronics and add to the fundamental understanding of the 1/f noise origin in any electronic device.