Purpose: Math word problem solving, a form of reading comprehension, is complicated by mathematical lexical ambiguity (e.g., the word "difference" can mean "dissimilarity" in everyday discourse but "the answer in a subtraction problem" in math). This study examined the role of mathematical lexical ambiguity in math word problem solving. Method: Lexically ambiguous math word knowledge, reading comprehension, vocabulary breadth, executive function (EF) skills, and math word problem solving were assessed in 521 3rd- to 5th-grade emergent bilingual (EB) and English monolingual (EM) students. Results: Students knew fewer math than common meanings of lexically ambiguous math words, and EBs knew fewer meanings than EMs. Multigroup path analysis indicated reading comprehension and lexically ambiguous math word knowledge contributed directly to math word problem solving and partially mediated the influence of EFs on math word problem solving. Conclusion: Consistent with the language function hypothesis, language skills supported math word problem solving directly and mediated the influence of EF skills on math word problem performance for both EBs and EMs alike. Our findings move the field forward by revealing a specific mechanism by which EF skills contribute to a particular aspect of content area reading comprehension.