Speaking to reporters in Washington from Harare, McGee blamed President Robert Mugabe's party which he said has "refused to act in good faith" to fulfill a September 15 power-sharing deal with the opposition.
"We have a very bad, dire political situation that's ... leading to a food and health emergency, manmade, in this country," McGee told reporters via satellite television hookup.
He said the capital's three main hospitals have closed and clinics in rural areas are reportedly turning away people in a country where health, education and other institutions are collapsing under the weight of runaway inflation.
And now there is a cholera epidemic.
He said he had received confirmed reports - though he did not cite the source - that 294 people have died from cholera among 1,200 confirmed cases and 2,500 unconfirmed cases of the water-borne disease.
"We're working with NGOs (non-government organizations) and local communities to try to provide water tablets, saline tablets, the things that are necessary to take care of the epidemic here in Zimbabwe," he said.
The death toll he gave is more than three times higher than the official toll of at least 90 dead, including three from across the border in South Africa.
McGee said he witnessed the "total look of hopelessness on people's faces" when he spent a day in the Zimbabwean countryside on his way back from South Africa recently.
"It's very, very grim," McGee said.
"There are a lot of distended bellies out there with small children. There are a lot of people picking absolutely sweet, but non-nutritious foods, off trees, trying to find anything to eat," he said.
"So, the situation in the countryside is bad, and we're starting to see the same thing happen here in the city," the ambassador added.
Some 1.5 million people are threatened with "food insecurity," he said, citing UN figures.
He said schools and universities are closing under the effects of runaway inflation of 210 million percent.
"So the school system has totally fallen apart, not only at the primary education level, at the secondary and also at the university level," he said.
Washington is contemplating new sanctions againt Mugabe's Zanu-PF party to force it to honor the agreement to share power with Morgan Tsvangirai's Movement for Democratic Change.
"We feel that unless something does happen in the very, very near future, we have no choice but to become more difficult and tougher on our sanctions," McGee said.
He said US sanctions targetting members of the Zimbabwean government are hurting them, citing cases where Zimbabweans studying in Australia and the United States have been forced to return to Zimbabwe.
For now, McGee said, Mugabe's grip on power is as firm as ever thanks to a system of political patronage that has survived the economic crisis and targeted sanctions.
"The ability of this government to continue to burp along is unbelievable. They do find ways to make certain that ... those who need payoffs, receive those payoffs," he added.