The mammary gland is a heterogeneous organ that modulates ductal morphogenesis and alveolar differentiation. Obesity is a significant risk factor for several cancers, including postmenopausal breast cancer. We and others have described an association between obesity and increased breast cancer growth. However, the effects of obesity on the mammary fat pad microenvironment (MFPME) remain understudied. Here, we investigated the effect of the Western Diet (WD) on immunocompetent female mice and on their MFPME. Our data suggest that the WD increased body, liver, and perigonadal white adipose tissue (pWAT) weight, as well as myeloid cell infiltration into these tissues. Interestingly, we did not find any significant change in CD4⁺ and CD8⁺ T cells in the liver, blood, and pWAT. NanoString data demonstrates that various cellular processes, including the complement system, innate immune system, phagocytic activity, immune metabolism, and NOD-like receptor (NLR) signaling, were upregulated in the MFPME of obese mice. RNA-Seq data suggest that WD significantly modulated MFPME physiology through regulation of gene expression, cellular processes, and signaling pathways. Further investigation is necessary to determine how WD-mediated changes in MFPME modulate breast cancer biology.