Background <p><i>Mycoplasma pneumoniae</i> is one of the causes of community-acquired pneumonia (CAP) with rare extrapulmonary complications, including haemolytic anaemia, persistent leukocytosis, Stevens‒Johnson syndrome, and meningoencephalitis. While these conditions are most common in children and young adults, typically those aged 5–20 years, this case report covers <i>Mycoplasma pneumoniae</i> infection with the very rare complication of haemolytic anaemia and persistent leukocytosis in middle-aged woman.</p> Case presentation <p>A 44-year-old woman with a history of high-grade fever for 10 days, fatigue, and headache came to emergency medicine. Laboratory evaluation revealed severe normocytic anaemia (Hb 5.3 g/dL), leukocytosis (42,000/µL), and thrombocytosis (564,000/µL). Cold agglutinin testing confirmed anti-I immunoglobulin M (IgM) mediated disease with high titres (1:64), and the serology was positive for M. pneumoniae IgM and immunoglobulin G (IgG) antibodies. She was treated with warmed red cell transfusions, intravenous methylprednisolone, and clarithromycin, followed by oral levofloxacin. Her condition improved significantly.</p> Rationale <p>This case highlights the diagnostic complexity in a woman patient with rare haemolytic anaemia in the context of atypical infections and the approach for their diagnosis and appropriate treatment. The coexistence of IgM-mediated autoimmune haemolytic anaemia and leukocytosis poses diagnostic and therapeutic challenges that are relevant to clinical practice. This case underscores the diagnostic and therapeutic challenges of cold agglutinin haemolysis secondary to <i>M. pneumoniae</i> infection.</p>