King Penguins breed on subantarctic islands between 45 and 55°S, at the northern reaches of Antarctica, as well as Tierra del Fuego, the Falkland Islands, and other temperate islands of the region. The total population is estimated to be 2.23 million pairs and is increasing. The largest breeding populations are on Crozet Island, with around 455,000 pairs, 228,000 pairs on the Prince Edward Islands, 240,000–280,000 on the Kerguelen Islands and over 100,000 on the South Georgia Islands. Macquarie Island has around 70,000 pairs. The non-breeding range is poorly known though presumably the subantarctic waters of the southern Indian, South Atlantic and Asian part of the Southern Ocean. Vagrant birds have been recorded from the Antarctic peninsula as well as South Africa, Australia and New Zealand.
The King Penguin is able to breed at three years of age, although the average age of first breeding is around 6 years. King Penguins are serially monogamous. They have only one mate each year, and stay faithful to that mate. However, fidelity between years is only about 29%.The long breeding cycle may contribute to this low rate. The King Penguin has an unusually prolonged breeding cycle, taking some 14–16 months from laying to offspring fledging. Although pairs will attempt to breed annually, they are generally only successful one year in two, or two years in three in a triennial pattern on South Georgia. The reproductive cycle begins in September to November, as birds return to colonies for a prenuptial moult. Those that were unsuccessful in breeding the previous season will often arrive earlier. They then return to the sea for three weeks before coming ashore in November or December. The female penguin lays one pyriform (pear-shaped) white egg weighing 300 g (? lb). It is initially soft, but hardens and darkens to a pale greenish colour. It measures around 10 × 7 cm (3.9 × 2.8 in). The egg is incubated for around 55 days with both birds sharing incubation in shifts of 6–18 days each. Hatching may take up to 2–3 days to complete, and chicks are born semi-altricial and nidicolous. In other words, they have only a thin covering of down and are entirely dependent on their parents for food and warmth. The young chick is brooded in what is called the guard phase, spending its time balanced on its parents' feet and sheltered by its pouch. During this time, the parents alternate every 3–7 days, one incubating while the other forages. This period lasts for 30–40 days before the chicks form crèches, a group of many chicks together. A penguin can leave its chick at a crèche while it fishes as a few adult penguins stay behind to look after them. Other varieties of penguins also practice this method of communal care for offspring. By April the chicks are almost fully grown, but lose weight by fasting over the winter months, gaining it again during spring in September. Fledging then takes place in late spring/early summer.
King penguins eat small fish, and squid and rely less than most Southern Ocean predators on krill and other crustaceans. Fish constitute 80–100% of the diet, except in winter months of July and August, when they make up only 30%. Lanternfish are the main fish taken. Slender escolar is also consumed. Cephalopods consumed include the hooked squid and the Sevenstar Flying.
The King Penguin's predators include birds and aquatic mammals; Skua species take small chicks and eggs, while the Snowy Sheathbill scavenges for dead chicks and unattended eggs. The Leopard Seal takes adult birds at sea. Orcas may also hunt king penguins.