West Antarctic ice shelves are rapidly melting and thinning, accelerating global sea-level rise. Limited glaciological and paleoceanographic evidence suggests that the continental shelf regions may have undergone a regime shift from a cold to a warm state during the past century, but no direct ocean observations have captured such a transition. Here, we synthesize ocean observations spanning from the late 19th century to the present, including newly-available data from 1898 through the 1980s that reveal the long-term evolution of the West Antarctic continental-shelf ocean. We identify three distinct phases: initial thin warm-water intrusions, thermocline shallowing, and intrusions of even warmer and lighter water masses. The first phase appears to have begun in the early twentieth century, driven by the large-scale shoaling of Circumpolar Deep Water. Our results provide direct observational evidence that continental-shelf ocean warming began in the early 20th century, which suggests that West Antarctic glacier retreat may also have begun earlier than is commonly thought. Early 20th-century warming in coastal West Antarctica represents a critical observational constraint for improving Earth system models in both hindcasts and future projections.