Background Cardiomyopathy is a known complication of organic acidemias but generally thought to be secondary to poor metabolic control. Methods Our patient was found through biochemical testing and Sanger sequencing to harbor an Icelandic founder mutation: NM_052845.4(MMAB):c.571C > T(p.Arg191Trp), leading to an early presentation (4 h after birth) of cblB‐type methylmalonic acidemia (MMA). Biochemical testing of this patient suggested B‐12‐responsiveness and thus the patient was treated with cyanocobalamin throughout life. Informed parental consent was obtained for this report. Results Our patient had three metabolic decompensations in her life (at birth, at 1 month, and at 5 months). The first decompensation was probably linked to stress of delivery, second to rhinovirus infection, and third by co‐infection of norovirus and enterovirus. At 3 months, the patient was noted to be tachypneic, although this was attributed to her underlying metabolic acidosis. At 5 months and 10 days, the patient was admitted with minor flu‐like symptoms but developed severe diarrhea in hospital and upon rehydration had cardiac decompensation and was found to have undiagnosed dilated cardiomyopathy. Although, patient was treated aggressively with dextrose, hemodialysis, levocarnitine, and vasoactive agents, there was limited response to medications to treat cardiac failure, and eventually the patient passed away before turning 6 months old. Conclusions Other than these three mild decompensations, patient had very good metabolic control, thus demonstrating that even without frequent metabolic decompensation, cardiomyopathy can be an observed phenotype in cblB‐type MMA even very early in life, suggesting that this phenotype may be independent of metabolic control. Early heart failure in a patient with methylmalonic acidemia (MMA). The patient succumbed to heartfailure due to dilated cardiomyopathy despite regular control as recommended for MMA patients.