The mallard is 56–65 centimetres long, has a wingspan of 81–98 centimetres, and weighs 0.9–1.2 kilograms. The breeding male is unmistakable, with a bright green head, black rear end and a yellowish orange (can also contain some red) bill tipped with black (as opposed to the dark brown bill in females). The female mallard is light brown, like most female dabbling ducks. However, both the female and male mallards have distinct purple speculum edged with white, prominent in flight or at rest (though temporarily shed during the annual summer moult). In non-breeding (eclipse) plumage, the drake becomes drab, looking more like the female, but still distinguishable by its yellow bill and reddish breast.
A noisy species, the male has a nasal call, the female has a “quack” stereotypically associated with ducks.
The mallard is a rare example of both Allen’s Rule and Bergmann’s Rule in birds. Bergmann’s Rule, which states that polar forms tend to be larger than related ones from warmer climates, has numerous examples in birds. Allen’s Rule says that appendages like ears tend to be smaller in polar forms to minimise heat loss, and larger in tropical and desert equivalents to facilitate heat diffusion, and that the polar taxa are stockier overall. Examples of this rule in birds are rare, as they lack external ears. However, the bill of ducks is very well supplied with blood vessels and is vulnerable to cold.
In captivity, domestic ducks come in wild-type plumages, white, and other colours. Most of these colour variants are also known in domestic mallards not bred as livestock but kept as pets, aviary birds, etc., where they are rare but increasing in availability.