Circulating hemocytes in the hemolymph constitute the backbone of innate immunity in the hard clam Mercenaria mercnearia. Hemocytes are also present in the extrapallial fluid (EPF), the space demarcated between the shell and the mantle tissue, which is the site of shell biomineralization. This study investigated the transcriptome, proteome, and function of hemocytes found in the EPF and hemolymph in the hard clam. Total and viable hemocyte counts were similar between EPF and hemolymph. Overexpressed genes in the EPF were found to have domains previously identified as being part of the “biomineralization toolkit” and involved in bivalve shell formation. Biomineralization related genes included chitin-metabolism genes, carbonic anhydrase, perlucin, and insoluble shell matrix protein genes. Overexpressed genes in the EPF encoded proteins present at higher abundances in the EPF proteome, specifically those related to shell formation such as carbonic anhydrases and insoluble shell matrix proteins. Genes coding for bicarbonate and ion transporters were also overexpressed, suggesting that EPF hemocytes are involved in regulating the availability of ions critical for biomineralization. Calcium content of hemocytes in the EPF were significantly higher than those in hemolymph, supporting the idea that hemocytes serve as a source of calcium during biomineralization. Overexpressed genes also contained domains such as C1q that have dual functions in biomineralization and immune response. The percent of phagocytic hemocytes was not significantly different between EPF and hemolymph. Together, these findings support the idea that hemocytes in EPF have dual functions of biomineralization and immunity.