Emerging evidence highlights the important impact of early-life exposures on cancer development later in life. The present study aimed to investigate the impacts of a high-fat diet in early life on the mammary microenvironment in relation to breast tumorigenesis. Forty-four female C57BL/6 mice were fed a low-fat diet (LF, 10 kcal% fat) or a high-fat diet (HF, 60 kcal% fat) for 8 weeks starting at ~4 weeks of age. Twenty-two mice were sacrificed immediately after an 8 week feeding, and the rest of mice were switched to a normal diet for maintenance (Lab Diet, #5P76) for additional 12 weeks. A panel of metabolic parameters, inflammatory cytokines, as well as tumorigenic Wnt-signaling target genes were analyzed. The HF diet increased body weight and exacerbated mammary metabolic and inflammatory status. The disrupted microenvironment remains significant to the later life equivalent to young adulthood (p < 0.05). Mammary Wnt-signaling was elevated right after the HF diet as indicated by the upregulated expression of its downstream genes, whereas it was surprisingly suppressed after switching diets (p < 0.05). In summary, HF-induced overweight/obesity in early life altered the mammary metabolic and inflammatory microenvironments in favor of breast tumorigenesis, although its overall impact to breast cancer later in life warrants further investigation.