Purpose: Tumour thrombus is a rare but critical manifestation of various malignancies, traditionally managed with open surgery or medical therapy. We present a three-patient case series demonstrating the role of mechanical thrombectomy using the Inari ClotTriever and FlowTriever systems for the diagnosis and treatment of intravascular tumour thrombus across multiple venous territories. Methodology: A retrospective analysis was performed on three patients with tumour thrombus involving the inferior vena cava (IVC), pulmonary arteries, and superior vena cava (SVC), who were managed between 2023 and 2025. All patients underwent percutaneous mechanical thrombectomy using Inari devices, with adjunctive intravascular ultrasound (IVUS) and digital subtraction angiography (DSA). Tissue samples obtained during thrombectomy were assessed histopathologically to confirm the diagnosis. Results: All procedures were successfully performed with no intra-procedural complications and low blood loss (50–400 mL). In Case 1, a level II (Mayo Classification) IVC tumour thrombus from renal cell carcinoma (RCC) was removed without IVC clamping. In Case 2, dual thrombectomy addressed both IVC and pulmonary tumour emboli in a Jehovah’s Witness patient, avoiding the need for blood products. In Case 3, the SVC tumour thrombus was managed under local anaesthesia, allowing for tissue diagnosis of small-cell neuroendocrine carcinoma while avoiding general anaesthesia and stenting. In all cases, flow restoration was confirmed with IVUS and DSA, and histological analysis enabled accurate tumour identification. Conclusion: Mechanical thrombectomy provides a safe and minimally invasive alternative for managing and diagnosing tumour thrombi, particularly in high-risk or anatomically challenging cases. The combination of thrombectomy devices, IVUS, and embolic protection facilitates both therapeutic and diagnostic goals. Larger studies are warranted to assess long-term outcomes and recurrence risk.