Sudden unexpected death in epilepsy (SUDEP) is the leading cause of death in patients with refractory epilepsy. Defects in central control of breathing are important contributors to the pathophysiology of SUDEP, and serotonin (5‐HT) system dysfunction may be involved. Here we examined the effect of 5‐HT neurone elimination or 5‐HT reduction on seizure risk and seizure‐induced mortality. Adult Lmx1b^f/f/p mice, which lack >99% of 5‐HT neurones in the CNS, and littermate controls (Lmx1b^f/f) were subjected to acute seizure induction by maximal electroshock (MES) or pilocarpine, variably including electroencephalography, electrocardiography, plethysmography, mechanical ventilation or pharmacological therapy. Lmx1b^f/f/p mice had a lower seizure threshold and increased seizure‐induced mortality. Breathing ceased during most seizures without recovery, whereas cardiac activity persisted for up to 9 min before terminal arrest. The mortality rate of mice of both genotypes was reduced by mechanical ventilation during the seizure or 5‐HT_2A receptor agonist pretreatment. The selective serotonin reuptake inhibitor citalopram reduced mortality of Lmx1b^f/f but not of Lmx1b^f/f/p mice. In C57BL/6N mice, reduction of 5‐HT synthesis with para‐chlorophenylalanine increased MES‐induced seizure severity but not mortality. We conclude that 5‐HT neurones raise seizure threshold and decrease seizure‐related mortality. Death ensued from respiratory failure, followed by terminal asystole. Given that SUDEP often occurs in association with generalised seizures, some mechanisms causing death in our model might be shared with those leading to SUDEP. This model may help determine the relationship between seizures, 5‐HT system dysfunction, breathing and death, which may lead to novel ways to prevent SUDEP.