A U.S. official said the drone strike occurred around 6 a.m. ET Saturday in a remote area of the Pakistan-Afghanistan border, southwest of the town of Ahmad Wal.
President Barack Obama said during his visit to Vietnam that the death of Mansour marks an "important milestone in our longstanding effort to bring peace and prosperity to Afghanistan."
The President, who also confirmed Mansour's death, added that it does not mark a "shift in our approach," stressing that the United States is not reentering the day-to-day combat operations.
Mansour was the target of the strike, and a second adult male combatant traveling with him in a vehicle also was likely killed, the official said.
"Mansour played a key leadership role in not only orchestrating the Taliban but orchestrating a variety of other organizations to include the Haqqani Network and al Qaeda who were perpetrating operations against not only U.S. forces but coalition forces and Afghan forces for a long period of time," Gen. Joseph Votel of U.S. Central Command said at a news conference in Amman, Jordan.
"He's an individual who has been in that structure for a long time. I'm glad he's gone."
Mansour was planning new attacks against U.S. targets in Kabul, a senior U.S. military official tells CNN Monday. In announcing the strike, the Pentagon said the Taliban had targeted and continued to target U.S. and coalition forces. This is the first time U.S. officials have cited new, specific threats against U.S. personnel in the Afghan capital city.
U.S. Secretary of State John Kerry said Mansour "was directly opposed to peace negotiations."
"This action sends a clear message to the world that we will continue to stand with our Afghan partners as they work to build a more stable, united, secure and prosperous Afghanistan," Kerry said at a news conference in Myanmar. "It is time for Afghans to stop fighting and to start building a real future together."
The United States notified Pakistan's government of the strike, Kerry said.
"We have had longstanding conversations with Pakistan and Afghanistan about this objective with respect to Mullah Mansour, and both countries' leaders were notified of the airstrike," he said. "And it is important for people to understand that Mullah Mansour, as I said a moment ago, has been actively involved in planning attacks in Kabul, across Afghanistan, presenting a threat to Afghan civilians and to the coalition forces that are there."
Pakistan: Strike a violation of sovereignty
Pakistani officials said they learned of the strike after it was carried out.
"While further investigations are being carried out, Pakistan wishes to once again state that the drone attack was a violation of its sovereignty, an issue which has been raised with the United States in the past as well," said a statement from Pakistan's Ministry of Foreign Affairs.
On Sunday, a senior Pakistani official told CNN that the strike on Mansour surprised his country.
The United States, Pakistan, Afghanistan and China have been working on a reconciliation process, and the official said there was an "understanding that (the) Taliban would not be targeted. The last meeting last week of this group the countries again issued a statement about pursuing a political settlement. So we aren't clear what led to the targeting of Mansour."
But the official would not say the strike was a violation of that understanding between the four countries.
Drone strikes are considered a violation of Pakistani sovereignty, but the official said Pakistan and the United States have "good cooperation" on military and intelligence. The official noted U.S. Gen. John Nicholson talked directly with Pakistan Army Chief of Staff Gen. Raheel Sharif, and Kerry called Pakistani Prime Minister Nawaz Sharif. When asked whether the attack hurt that cooperation, the official said no.
Afghan officials also confirmed the killing.
Abdullah Abdullah, Afghanistan's chief executive, tweeted that Mansour was the victim of a drone strike that targeted his car in the Dahl Bandin area of Quetta in Pakistan.
The strike was carried out by multiple unmanned aircraft operated by U.S. Special Operations forces. There was no collateral damage, the first U.S. official added.
President Barack Obama authorized the strike.
Key figure in Taliban
The Pentagon confirmed the strike, saying Mansour had been "actively involved with planning attacks against facilities in Kabul and across Afghanistan, presenting a threat to Afghan civilians and security forces, our personnel and coalition partners."
"Mansour has been an obstacle to peace and reconciliation between the government of Afghanistan and the Taliban, prohibiting Taliban leaders from participating in peace talks with the Afghan government that could lead to an end to the conflict," Pentagon press secretary Peter Cook said in a statement.
Sen. Lindsey Graham of South Carolina, one of the more hawkish Republicans in the Senate, welcomed the news and urged Obama not to withdraw troops from Afghanistan "until conditions on the ground permit their withdrawal."
"I'm glad to hear we decided to bring the leader of the Afghan Taliban, Mullah Akhtar Mansour, to justice. Mansour has terrorized the Afghan people as well as coalition forces," Graham said in a statement. "I appreciate President Obama for authorizing the attack. And job well done to the members of our military and intelligence communities who carried out the mission."
Sen. Bob Corker, a Tennessee Republican and chairman of the Senate Foreign Relations Committee, said, "If verified, the death of Taliban leader Mullah Akhtar Mansour would be an important victory in the fight against terror and welcome news to our military personnel in Afghanistan and the Afghan government."
Rise through the ranks
The Taliban revealed last year that Mansour assumed command
following the death of longtime leader Mullah Mohammed Omar, who died in Pakistan in 2013.
Mansour formerly headed the leadership council of the Taliban and Islamic scholars, also known as the Quetta Shura, which includes longtime leaders who direct the Taliban's operations from Pakistan's Balochistan province, according to the Jamestown Foundation
, a global research and analysis group.
According to the U.N. Security Council sanctions list, Mansour had been the Taliban's minister of civil aviation and transportation and was considered "a prominent member of the Taliban leadership."
"He was repatriated to Afghanistan in September 2006 following detention in Pakistan. He is involved in drug trafficking and was active in the provinces of Khost, Paktia and Paktika in Afghanistan as of May 2007. He was also the Taliban 'Governor' of Kandahar as of May 2007," the U.N. document said
He was an active recruiter in the Taliban's fight against the Afghan government, the United Nations said.
Taliban leadership gathered in Quetta, Pakistan, to choose a successor for the insurgent group following the death of Mansour, the deputy leader of a Taliban splinter group told CNN.
"On Sunday, a number of Mullah Akhtar Mohammad Mansour's people (Taliban senior members) got to gather in Quetta of Pakistan, but after a full day of talking, they couldn't make a decision on whom to choose as a successor for Mullah Mansour," according to Mawlawi Abdul Manan Niazi, deputy leader of the Taliban splinter group led by Mawlawi Mohammad Rasool.
Strain on U.S.-Pakistan relationship
Killing Mansour will probably only throw the Taliban into temporary disarray, U.S. intelligence officials told CNN.
But the big implication is for Pakistan, which U.S. intelligence knows is providing training and support for the Haqqani network -- the most ruthless and effective faction of the Taliban -- which is active in carrying out attacks on American and Afghan government forces.
While Pakistan is upset about a strike in its territory, U.S. intelligence believes it should have to explain why a Taliban leader was in its country.
U.S. officials said they are watching closely to see if the killing of Mansour leads to Sirajuddin Haqqani, head of the Haqqani network, taking over as leader of the Taliban, and whether Pakistan continues providing support to the Haqqani network.
In Congress, frustration with Pakistan is rising over its support for the Haqqani network, and the annual U.S. aid package to the country is facing strong resistance.