Designed DNA-DNA interactions are investigated for their ability to modulate protein packing within single crystals of mutant green fluorescent proteins (mGFPs) functionalized with a single DNA strand (mGFP-DNA). We probe the effects of DNA sequence, length, and protein-attachment position on the formation and protein packing of mGFP-DNA crystals. Notably, when complementary mGFP-DNA conjugates are introduced to one another, crystals form with nearly identical packing parameters, regardless of sequence if the number of bases is equivalent. DNA complementarity is essential, because experiments with non-complementary sequences produce crystals with different protein arrangements. Importantly, the DNA length and its position of attachment on the protein markedly influence the formation of and protein packing within single crystals. This work shows how designed DNA interactions can be used to influence the growth and packing in X-ray diffraction quality protein single crystals and is thus an important step forward in protein crystal engineering.