The Kabul ceremony followed six months of deadlock amid a bitter dispute over electoral fraud and a recount of votes.
Under a US-brokered unity deal Mr Ghani shares power with runner-up Abdullah Abdullah who becomes chief executive.
The Taliban called the deal a “US-orchestrated sham”. A blast near Kabul airport killed at least seven people.
Interior ministry spokesman Sediq Sediqi said a suicide bomber attacked a security checkpoint on the airport road leading to the US embassy.
Four members of the Afghan security forces and three civilians were killed, and a number of other people were wounded, he said.
Constitutional changes
At the swearing-in ceremony attended by up to 100 dignitaries at the presidential palace in Kabul, Mr Ghani took an oath to abide by the constitution.
He said he would work for long-term peace, promised to tackle corruption and said constitutional changes were needed.
In his long first speech after being sworn in as the new Afghan president, Ashraf Ghani promised reform, development, an end to poverty, measures against corruption, and a clean-up of the judiciary.
But he knows that he can do nothing without security. In a year when the Taliban are conducting their biggest offensives since they fell from power in 2001, he said Afghan people were tired of war and wanted peace.
He talked about the crisis in Syria and Iraq, and said that Afghanistan was an Islamic country that understood about jihad after Islamic guerrillas defeated the Russians in the 1980s. He said the country did not need any lessons from foreign fighters.
The former World Bank economist has ambitious plans for change, but now needs to make his coalition work, with his defeated election rival Abdullah Abdullah.
Mr Ghani praised the country’s “first democratic transfer of power” and also spoke warmly of his rival, and now partner in government, Dr Abdullah.
Dr Abdullah, who takes on the new chief executive role with prime-ministerial powers, said the two leaders would work together “for a better future with trust and honesty”.
Earlier outgoing President Hamid Karzai, who has been leader since the US-led invasion in 2001, called for people to support the new government.
The BBC’s David Loyn in Kabul says security in the capital is tight, with few people on the streets and shops closed.
- Previously an academic in the US, and worked for the World Bank
- Served as finance minister from 2002-2004 under President Hamid Karzai
- He fell out with President Karzai in 2004 and became Chancellor of Kabul University, where he was seen as an effective reformer
- He has been a vocal critic of the way international aid money has been wasted in Afghanistan
- He has a well-known reputation for having a quick temper
The first thing the government is expected to do is to sign a deal that will see US troops remain in Afghanistan after the end of this year – a move previously opposed by Mr Karzai. All Nato combat troops pull out this year.
In other violence on Monday, a suicide car bomb attack on a government compound in the eastern province of Paktia killed at least eight people, officials say.
The Taliban said they carried out the raid in Zurmat district in which a number of security personnel died. Officials say four attackers were also killed.
On Friday the Taliban overran a strategic district in another eastern province, Ghazni, highlighting some of the many challenges facing Mr Ghani and his security forces.
Following the election earlier this year, US Secretary of State John Kerry helped broker a comprehensive audit of all eight million votes after the results were disputed.
The audit was completed this month but the final tallies and the official result have not been made public amid fears over unrest.
Afghanistan’s election commission confined itself to declaring Mr Ghani the winner in a statement earlier this month.
Both sides had accused the other of fraud following the election and months of uncertainty have damaged the economy and heightened insecurity.
BBC.
