Mammary gland tumors (MGT) are the most common neoplasms in intact female dogs and represent the third most frequent tumor type in female cats. Imaging plays a central role in the diagnosis, staging, and follow-up of these neoplasms. This review summarizes the diagnostic imaging features of canine and feline MGT and the advantages and limitations of each modality. Radiography and computed tomography (CT) are mainly used to detect pulmonary and abdominal metastases, while ultrasonography provides key information about primary tumor size, margins, echogenicity, vascularization, and elasticity. Advanced ultrasound techniques, such as Doppler, contrast-enhanced ultrasonography (CEUS), and elastography, improve differentiation between benign and malignant lesions. Magnetic resonance imaging (MRI) offers excellent soft-tissue detail and can assist in preoperative planning, whereas nuclear medicine techniques, including scintigraphy and positron emission tomography (PET), allow functional assessment and the early detection of micrometastases. Although histopathology remains the diagnostic gold standard, imaging is indispensable for characterizing primary lesions, evaluating metastatic spread, guiding sampling, and monitoring therapeutic outcomes in small animal oncology. By integrating and comparing the main imaging modalities applied to canine and feline mammary tumors, this review underscores their complementary roles in improving diagnosis, staging, and therapeutic decision-making in small animal oncology.