Adult male Leopard Seals are 2.8 to 3.3 m long and weigh up to 300 kg. Adult females are 2.9 to 3.6 m, with very large animals possibly reaching 3.8 m, and weights of 260 to upwards of 500 kg. Pups are 1.0 to 1.6 m in length and weigh 30 to 35 kg at birth. The age at sexual maturity is probably four years for females and 4.5 years for males. Longevity is estimated to be over 26 years (Kooyman 1981, Rogers 2009).

At sea and on the ice, Leopard Seals tend to be solitary. Pups are born on sea ice from early November to late December and the period may be as long as early October to early January (Southwell et al. 2003). Births at South Georgia occur from late August to the middle of September. Pups are probably weaned at four weeks old, and female oestrous occurs at or shortly after weaning. Unlike Crabeater Seals, male Leopard Seals do not haul out with female-pup pairs. Mating is believed to occur in the water, but has never been observed.

Leopard Seals are well known for preying upon penguins. However, their diet is in reality highly varied and changes with seasonal and local abundance of prey. Leopard Seals will consume krill, fish, squid, penguins, a variety of other types of seabirds, and juvenile seals including Crabeater, Southern Elephant and Fur Seals (Rogers 2009, Southwell et al. 2012). In the case of Antarctic Fur Seals, it has been proposed that predation of Leopard Seals on pups might have a significant role in driving population dynamics of the prey species in certain areas (Schwarz et al. 2013). Most prey are caught in the water. Penguins are usually held in the Leopard Seal's teeth by one end and slung in an arc with a rapid snap of the head and neck and smashed on the surface of the water until they are torn open. Smaller pieces are then swallowed. Young, newly fledged naïve penguins are most vulnerable, but adult birds are taken as well. Leopard Seals patrol and regularly station themselves just off Penguin colonies and wait to ambush and chase animals transiting to and from the colonies (Rogers and Bryden 1995).  Krill may be a seasonally important prey (Lowry et al. 1988).

There have been few studies of Leopard Seal diving. Two seals tagged with satellite-linked dive recorders dove mostly to 10-50 m, and the deepest dive recorded was 304 m (Nordøy and Blix 2009).

Leopard Seals are widely distributed in Antarctic and sub-Antarctic waters of the Southern Hemisphere, occurring from the coast of the Antarctic continent northward throughout the pack ice and at most sub-Antarctic islands. There is a seasonal presence of juveniles at Kerguelen and Macquarie Islands with the greatest numbers being sighted in September and October (Rounseveld and Eberhard 1980, Borsa 1990). Vagrants regularly reach warm temperate latitudes. Leopard Seals haul out on ice and on land, often preferring ice floes near shore when they are available (Kooyman 1981).

The Leopard Seal is not listed as endangered or threatened on any national Red List. Under the Antarctic Treaty's Convention for the Conservation of Antarctic Seals, the global quota set for Leopard Seals is 12,000 animals annually, but currently there is no harvesting.