<p>Three decades after the rise of mixed electoral systems (MESs), initial expectations of their benefits are mostly tempered. A key reason for the disappointment is the contamination of electoral competition in single-member districts, as parties run nonviable candidates. Strategic incentives to run everywhere vary between subtypes of MESs, being strongest in one-vote MESs and weakest in seat-linkage MESs. However, electoral systems do not emerge in a vacuum, and pre-reform party competition can also decisively influence the post-reform strategic considerations of parties, candidates, and voters. This paper explores the enduring impact of party-system legacies on contamination of single-member-district competition in MESs. I predict relative stability of previous patterns of party competition (in multiparty, two-party, and dominant-party systems), except after critical junctures (one-party-system breakdowns). The empirical analysis covers 26,000 + single-member districts across 141 elections in twenty-five countries spanning seven decades, examining key dependent variables including the effective number of candidates, the ratios of second-to-first loser and third-and-lower-to-first loser, and the vote share of third- and lower-placed candidates. I also assess various electoral, institutional, and contextual factors.</p>