Tomatoes are a major global food staple but Phytophthora infestans (an Oomycete) causes late-blight, a devastating disease that precludes commercial tomato production from moist temperate areas such as the United Kingdom and Northern Europe. We dissected the genetic architecture of resistance to late-blight as well as traits that improve yield and fruit quality in a tomato cross between a popular breeding, line NC 2 CELBR, which produces large fruits, and an heirloom cultivar called ‘Koralik’ which produces small, sweet fruits. We used an F2 mapping population to identify quantitative trait loci (QTL) for phenotypes including number of fruits, size of fruits, total crop yield, and soluble solids content in two different environments. Surprisingly, we found very few QTLs shared between the two environments, underscoring the importance of the local environment and genotype-by-environment interactions. We also assayed the virulence of three different isolates of P. infestans to identify QTLs that confer some resistance to the pathogen. We found nine crop-related QTLs and two QTLs for late-blight resistance-related phenotypes. One late-blight resistance QTL was inherited from Koralik (Chromosome 11, 70.2–83.5 cM) and it probably represents an undiscovered source of late-blight resistance. Yield QTLs were also located on chromosome 11 where Koralik alleles increase fruit number and yield, and adjacent regions decrease fruit size. On Chromosome 9, Koralik alleles increase fruit sweetness (Brix) by 25%. These results indicate that Koralik is a valuable donor parent that can be used by tomato breeders in targeted breeding strategies for fresh market tomatoes.