Susan Rice has called the UN Security Council's inaction on the Syrian war "a stain" on the body, in final remarks as US ambassador to the organisation.
She criticised Russia and China for vetoing three resolutions that would have increased pressure on Syrian President Bashar al-Assad.
But the US envoy said the post had been "the best job I've ever had".
Ms Rice is leaving the ambassadorship to become President Barack Obama's national security adviser.
Mr Obama has nominated Samantha Power, a Pulitzer Prize-winning author and former White House adviser, to replace her. Ms Power must now be confirmed by the US Senate.
'Moral, strategic disgrace'
The departing US ambassador described her time at the UN as "a remarkable period", but said she regretted more was not done to stem the bloodshed in Syria.
"I particularly regret that the Security Council has failed to act decisively as more than 90,000 Syrians have been killed and millions more displaced," she said.
"The council's inaction on Syria is a moral and strategic disgrace that history will judge harshly."
On Wednesday the UK-based activist group the Syrian Observatory for Human Rights said the death toll was even higher than the UN figure used by Ms Rice, putting it at 100,191.
More than 36,000 of those killed in the 27-month conflict were civilians, it said, including more than 3,000 women and more than 5,000 children under 16.
It cautioned that the true number of combatants killed could be twice what it had documented due to both sides' secrecy in reporting casualties.
Ms Rice said the council's failure to act could not be laid at the feet of the United States.
"I don't know how in any circumstance one could ascribe that to a failure of US policy or US leadership," she said, "when the vast majority of the council was ready and willing to move ahead."
Russia and China used their Security Council veto powers once in 2011 and twice in 2012 to prevent the body from adopting resolutions that condemned the violence in Syria, demanded an end to human rights violations by Syrian government forces, and threatened non-military sanctions.
The Russians criticised the resolutions as tantamount to taking sides in a civil war.
Despite Ms Rice's criticism of Russia on Syria, she said it was not inevitable that "complex and multifaceted" relations between the two countries should sour.
"On issues as important as Iran and North Korea, and many others, we have been able to find common ground," she said.