As the fierce battle with cancer is now expanding to companion animals, effective treatment of canine mammary carcinomas (CMT), as the most frequently diagnosed tumor in intact dogs, is becoming a crucial issue. Although many studies have been carried out concerning the clinical application of mammary tumor biomarkers, no ideal biomarker has yet been identified in CMT. Therefore, in this work, we develop EDIL3 as a CMT biomarker having significantly higher expression levels in CMT samples compared to those in controls in GSE13754, GSE22516 and GSE25586 datasets, which suggest that EDIL3 is a gene related to tumorigenesis. We also validate the significantly high expression levels of EDIL3 in CMT samples using our sequencing canine samples. ROC curves analysis showed that in comparison with HER2 reported as predictive factor for CMT patients, EDIL3 exhibits stronger power for CMT recognizing. Moreover, we also find that low expression levels of EDIL3 are associated with advanced grade status in CMT, which indicate a negative correlation between EDIL3 and CMT development. GSEA is employed to unveil the underlying mechanism of this interesting function of EDIL3 in CMT development, and it suggests that the expression level of EDIL3 is related to immunity pathway. Finally, CIBERSORT analysis is employed in this study in order to further explore the relationship between EDIL3 and immunity in CMT, and it unveils that EDIL3 has stably positive correlation with follicular helper T cells and negative correlation with NK resting cells in CMT. Our study develops EDIL3 as a biomarker for assisting CMT distinction, highlighting the relationship of EDIL3 with the infiltrations of follicular helper T cells and NK resting cells, which could be a new potential therapy target for CMT and provide bioinformatics basis for later clinical experiment validation.