Mass balance estimates of the East Antarctic ice sheet over the satellite era have a high degree of uncertainty, yet are of such significance that they can partially or even entirely offset the ice loss at the margins of the Antarctic Ice Sheet that contributes to global sea level rise. Therefore, it is imperative to improve our understanding of snowfall variability across time and space in East Antarctica. Here we present a new millennial-scale, annual snow accumulation history from the Mount Brown South (MBS) ice core, a semi-coastal record from 2,084 m elevation on the boundary of Wilhelm II and Princess Elizabeth Land in East Antarctica. We used a range of methods to test the best derivation of annual accumulation rates, and to ensure best practice in determining both density correction and layer thinning with depth. The MBS annual accumulation record spans 873–2017 CE, and displays a high degree of variability across time scales. The long-term mean snow accumulation rate at MBS is 0.276 ± 0.08 (1 standard deviation) m yr^−1 ice equivalent, which is almost 10 % lower than the mean accumulation rate in the satellite era (1979–2017) of 0.302 ± 0.08 m yr^−1 ice equivalent. While mean annual accumulation during the satellite era is within the range of natural variability, the full record exhibits substantial decadal to centennial-scale variability, including two notable low accumulation periods between CE 1127–1321 (195 years) and 1754–1879 (126 years), where accumulation was 12 % lower than the long-term mean, and 20 % lower than satellite era mean. While an ice core record is a point-based rather than spatially resolved estimate of annual accumulation, these results raise implications for future mass balance assessments in this region that assume the satellite era represents the natural range of variability.