At the time of Lambert's return to Leicester, his weight began to increase steadily, even though he was athletically active and, by his own account, abstained from drinking alcohol and did not eat unusual amounts of food. In 1805, Lambert's gaol closed. By this time, he weighed 50 stone (700 lb; 320 kg), and had become the heaviest authenticated person up to that point in recorded history. Unemployable and sensitive about his bulk, Lambert became a recluse.
In 1806, poverty forced Lambert to put himself on exhibition to raise money. In April 1806, he took up residence in London, charging spectators to enter his apartments to meet him. Visitors were impressed by his intelligence and personality, and visiting him became highly fashionable. After some months on public display, Lambert grew tired of exhibiting himself, and in September 1806, he returned, wealthy, to Leicester, where he bred sporting dogs and regularly attended sporting events. Between 1806 and 1809, he made a further series of short fundraising tours.
In June 1809, he died suddenly in Stamford. At the time of his death, he weighed 52 stone 11 pounds (739 lb; 335 kg), and his coffin required 112 square feet (10.4 square metres) of wood. Despite the coffin being built with wheels to allow easy transport, and a sloping approach being dug to the grave, it took 20 men almost half an hour to drag his casket into the trench, in a newly opened burial ground to the rear of St Martin's Church. While others have since overtaken Daniel Lambert's record as the heaviest person in history, he remains a popular character in Leicester, and in 2009 was described by the Leicester Mercury as "one of the city's most cherished icons".
Daniel Lambert was born at his parents' house in Blue Boar Lane, Leicester, on 13 March 1770. His father, also named Daniel Lambert, had been the huntsman to Harry Grey, 4th Earl of Stamford, and at the time of his son's birth was the keeper of Leicester's gaol. The eldest of four children, Daniel Lambert had two sisters, and a brother who died young.
At the age of eight he was a keen swimmer, and for much of his life he taught local children to swim. Lambert's paternal uncle—like his father—also worked with animals, but as a professional gamekeeper; his maternal grandfather was a breeder of champion fighting cocks. Lambert grew up with a strong interest in field sports, and was particularly fond of otter hunting, fishing, shooting and horse racing. From his early teens, Lambert was a keen sportsman and by his late teens he was considered an expert in the breeding of hunting dogs.