Agriculture Secretary Ann Veneman told a Washington press conference on Tuesday a sample from the suspect cow had been sent to Britain for tests.
She said the US authorities remained "confident" in the safety of US beef. The farm where the diseased cow was slaughtered will be quarantined. Veneman said the "risk to human health was minimal" but safety precautions would be taken. The diseased cow was identified as a "downer" animal- one that is too sick to walk.
Scientists believe humans can be infected with the brain-wasting disease by eating diseased meat.
Mad cow disease, also known as bovine spongiform encephalopathy (BSE), had previously not been found in the United States, but it devastated the European cattle industry in the 1990s.
Washington created a "response plan" to deal with any cases discovered within the United States, said the agriculture secretary. This plan has been activated.
The US cattle industry has long feared an outbreak of mad cow disease, which could result in billions of dollars of losses.
On 20 May, Canada confirmed that one Alberta cow, which was slaughtered in January, had BSE. The disease has been widespread in Europe and has been linked to about 130 human deaths, mostly in Britain.
The discovery of the sick Canadian cow triggered an immediate halt of Canadian meat exports by most countries as a precaution.
Because of concerns over mad cow disease, the European Union in 1994 banned the use of mammalian meat and bone meal in cattle feed, but it has allowed the products to be used in feed for other animals like chickens, pigs and fish.