Lithium is a potent psychotherapeutic agent that has gained wide acceptance in paediatrics, especially as adjunct treatment for severe behavioural, anxiety, and attention-deficit hyperactivity disorders, along with bipolar conditions. Its cardiac toxicity has been well-documented in adults; however, information is limited regarding lithium’s effects on the heart in children. Therefore, paediatric cardiologists following-up children on lithium therapy should be cognizant of the cardiac side-effects and pathophysiology associated with this drug. In this manuscript, we used an illustrative case of a child who presented with lithium poisoning, in order to highlight adverse clinical manifestations that can arise from this medication. The cardiac cell membrane is thought to be the primary site of lithium’s action. Thus, we reviewed lithium’s effects on membrane electrogenic pumps and channels involved in the distribution and passage of sodium, potassium, and calcium across the sarcolemma, as these ions, and their associated currents, are the primary determinates of the action potentials underlying auto-rhythmicity and contractile activity of the heart.