The present work identifies a so-far overlooked bias in sequential impression formation. When the latent qualities of competitors are inferred from a cumulative sequence of observations (e.g., the sum of points collected by sports teams), impressions should be based solely on the most recent observation because all previous observations are redundant. Based on the well-documented human inability to adequately discount redundant information, we predicted the existence of a cumulative redundancy bias. Accordingly, perceivers’ impressions are systematically biased by the unfolding of a performance sequence when observations are cumulative. This bias favors leading competitors and persists even when the end result of the performance sequence is known. We demonstrated this cumulative redundancy bias in 8 experiments in which participants had to sequentially form impressions about the qualities of two competitors from different performance domains (i.e., computer algorithms, stocks, and soccer teams). We consistently found that perceivers’ impressions were biased by cumulative redundancy. Specifically, impressions about the winner and the loser of a sequence were more divergent when the winner took an early lead compared with a late lead. When the sequence ended in a draw, participants formed more favorable impressions about the competitor who was ahead during most observations. We tested and ruled out several alternative explanations related to primacy effects, counterfactual thinking, and heuristic beliefs. We discuss the wide-ranging implications of our findings for impression formation and performance evaluation.