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<div>The oomycete <i>Phytophthora infestans </i>is an important pathogen on potato and tomato crops worldwide. It primarily reproduces asexually and forms clonal lineages. We compared the population structure of 18 of the 24 documented US lineages to lineages from Europe, South America, and Mexico. Structure analysis, neighbor joining trees, and principal component analyses of 12 microsatellite loci indicated that many recent US genotypes (US-7, US-8, US-11, US-22, US-24) shared significant allelic diversity with lineages from Mexico, while the US-1 lineages clustered with US-1 isolates from Peru. The US-8, US-14, and US-24 lineages, predominantly virulent on potato, and the US-21 and US-22 lineages, predominantly virulent on tomato, formed two distinct and separate monophyletic clades. The US-23 lineage, currently the most prevalent lineage detected in the US, shared allelic diversity primarily with isolates from Bolivia and Brazil and not Mexico. Mexican lineages showed evidence of multiple ancestral recombination events. A survey of the presence of RXLR effector <i>PiAVR2</i> across all samples revealed the presence of lineages that carried either <i>PiAVR2</i>, its resistance-breaking variant <i>PiAVR2-like</i>, or both, suggesting lineages have experienced different levels of selection to the <i>R2 </i>gene in potato. These findings suggest populations of <i>P. infestans</i> in the US are the result of introductions from both South America and Mexico.</div>