Introduction: Light-chain (AL) amyloidosis secondary to multiple myeloma (MM) is a rare but fatal condition requiring early diagnosis. We present a complex case of MM-associated AL amyloidosis with cardiac, renal, and gastrointestinal involvement, complicated by a history of medullary thyroid carcinoma. Case Presentation: A 60-year-old Bangladeshi male with hypothyroidism (post-thyroidectomy for medullary carcinoma) and peptic ulcer disease presented with bilateral leg edema, frothy urine, fatigue, and 8 kg weight loss over 2 months. Examination revealed moderate anemia, pitting edema, and hypoalbuminemia (3.15 g/dL) with nephrotic-range proteinuria (6.75 g/day). Laboratory studies showed monoclonal IgG lambda protein (kappa/lambda ratio: 0.22), elevated NT-proBNP (3900 pg/mL), and bone marrow plasmacytosis (25–30%). Renal biopsy confirmed amyloidosis with lambda-dominant deposits; echocardiography demonstrated restrictive cardiomyopathy. Diagnosis: Systemic AL amyloidosis with MM, involving the kidneys (nephrotic syndrome), heart (restrictive cardiomyopathy), and likely gastrointestinal tract (chronic constipation, weight loss). Interventions: Supportive care (diuretics, PPI, thyroxine) was initiated. Chemotherapy for MM/AL amyloidosis was planned but delayed due to the patient’s deteriorating clinical status. Outcomes: The case highlights the rapid progression of multisystem amyloidosis, emphasizing the need for early suspicion in patients with nephrotic syndrome, monoclonal gammopathy, and cardiac dysfunction. Conclusion: This case underscores the diagnostic complexity of MM-associated AL amyloidosis, particularly with concurrent malignancies (medullary thyroid carcinoma). A high index of suspicion, timely organ biopsies, and multidisciplinary management are critical to improving outcomes.