Colleagues said he had been frustrated at his lack of independence but Danforth made no mention of that in his resignation letter to President George Bush, saying he wanted to spend more time with his family.
"Forty-seven years ago, I married the girl of my dreams, and at this point in my life, what is most important to me is to spend more time with her," he said in a 22 November letter to Bush distributed to reporters on Thursday.
Some ex-colleagues said Danforth, a former senator from Missouri, had recently expressed frustration at not being more of an independent actor and having to check often with Washington.
White House surprise
The news came as a "complete surprise", one US official said, speaking on condition of anonymity.
"Forty-seven years ago, I married the girl of my dreams, and at this point in my life, what is most important to me is to spend more time
outgoing US ambassador to UN
In a speech last month in St Louis, Danforth said that as a former senator, he was not accustomed to having a policy statement vetted by State Department bureaucrats and
transformed into "mush" before he could issue it.
"It creates some practical problems," he told alumni at Washington University.
He has commented little on corruption in the UN oil-for-food humanitarian programme, which some US legislators have blamed solely on UN Secretary-General Kofi Annan.
He said only that everyone hoped investigations by the United Nations and congressional panels would bring out the truth.
Danforth, 68, began serving at the UN on 1 July, succeeding John Negroponte, who was appointed US ambassador to Iraq.
He is an ordained Episcopal minister who represented Missouri in the US Senate for 18 years.
Shortly before being named the 25th American ambassador to the United Nations, Danforth served as the Bush administration's peace envoy to Sudan for nearly three years.
Danforth worked hard to put an
end to the Sudanese civil war
Two weeks ago, he took the entire Security Council to Nairobi to pressure the Sudan government and rebel opponents in the south to end Africa's longest running civil war.
The two sides signed a pledge to complete peace talks by 31 December, with all 15 Security Council members as witnesses.
Elected in 1976 as a senator, Danforth left politics 10 years ago and returned to St Louis, where he joined a law firm.
But he has accepted several public assignments and headed the 1999-2001 inquiry into the Branch Davidian stand-off involving an obscure religious cult in Waco, Texas.
Danforth was born in 1936, the grandson of the founder of the cereal and grain empire Ralston Purina.
He graduated from Princeton University, and received both law and divinity degrees at Yale before joining a Wall Street law firm. He became Missouri's attorney-general in 1968.
Despite his religious background, Danforth has opposed organised prayer in schools and as a party moderate opposed a constitutional amendment to ban burning of the US flag.