He promised to be the "president of all Angolans" and to open dialogue after the electoral commission announced the results, which saw the opposition make large gains.
"This a victory for Angola and Angolans in general," Lourenco, 68, said in his inaugural address shortly after the unveiling of the result of the August 24 ballot.
"This vote was a vote of confidence, which gives us the immense responsibility of promoting dialogue and social consultation".
The National Electoral Commission (CNE) reported the People's Movement for the Liberation of Angola won 51.17 percent of the ballots against 43.95 percent for the main challenger, the National Union for the Total Independence of Angola (UNITA).
Despite the victory, the outcome -- the tightest in Angola's history -- marked a record low for the MPLA and might yet end up in court after UNITA had earlier rejected provisional results.
Several members of the electoral commission did not sign off on the final tally, poll officials said on Monday.
The MPLA has traditionally wielded control over the electoral process as well as state media, and opposition and civic groups had raised fears of voter tampering.
UNITA leader Adalberto Costa Junior, 60, last week called for an international panel to review the count.
International observers have raised some concerns including questions over the electoral roll and biased reporting by state-owned television, but most said voting was peaceful and well organised.
The MPLA, a former Marxist liberation movement, has ruled Angola for nearly half a century since independence from Portugal in 1975.
But it has seen a steady decline in support over recent elections.
While it romped to victory with 71.84 percent of the vote in 2012, it only garnered 61 percent five years later.
UNITA scored 26.67 percent in 2017 elections and contested the official count.
Alex Vines, of the UK-based think tank Chatham House, said that while UNITA was likely to challenge the count also this time, the former rebel movement had reasons to be happy.
"It's an amazing result for UNITA when you think that 20 years ago, they were defeated on the battlefield," he said.
"Politics will have to change in Angola now. There's going to have to be the politics of compromise," he said.
The results gave the MPLA 124 of the 220 parliamentary seats up for grabs while UNITA won 90.
Turnout was low, with only about 45 percent of those registered casting their ballots, which pointed to a growing disillusionment with politics, said Vines.
The United States on Monday called on all parties "to express themselves peacefully and to resolve any grievances in accordance with applicable legal processes".
"We will continue to closely follow the electoral process," the State Department said in a statement before the final results were announced.
The latest election has been overshadowed by a struggling economy, inflation, poverty, drought and the death of Lourenco's predecessor Jose Eduardo dos Santos.
Dos Santos was buried in Luanda at a solemn funeral on Sunday.
The opposition has proved popular in urban areas, winning in the capital Luanda and among youth disaffected with the ruling party.
Angola is Africa's second-largest crude producer, but the oil bonanza has been accompanied by corruption and nepotism.
Lourenco, a former general educated in the Soviet Union, was first elected in 2017.
He is credited with far-reaching reforms since taking power, including boosting financial transparency, tackling graft, and promoting business-friendly policies to lure foreign investors.
But critics say his anti-graft crusade has been one-sided and aimed at settling political scores, targeting children and cronies of dos Santos.
His economic reforms have also so far failed to translate into better living conditions for most Angolans, critics say.
"With this vote of confidence, it is time to continue the reforms necessary to make Angola a more prosperous and more developed country," Lourenco said, promising to pay particular attention "to the expectations of young people".