The 71-year-old southerner took over from a 80-year-old northerner as Africa’s most populous nation faces a sea of economic troubles and security challenges.
“As president of the Federal Republic of Nigeria I will discharge my duties and perform my functions honestly to the best of my ability, faithfully and in accordance with the constitution,” Tinubu said in a live broadcast from the capital Abuja’s Eagle Square.
Foreign leaders and representatives present at the ceremony included presidents Cyril Ramaphosa of South Africa, Paul Kagame of Rwanda and Nana Akufo-Addo of Ghana as well as delegations from the US, Britain and China.
Kashim Shettima was sworn in as vice president, taking over from Yemi Osinbajo.
The ruling party duo were declared winners of the February 25 election, gaining the highest number of votes — 8.8 million — and picking up the required number of ballots across two-thirds of Nigeria’s states.
But Tinubu only garnered just over a third of the total vote, leaving him with a weak mandate, some analysts say.
The poor showing reflects “widespread disenchantment with his party’s record… along with misgivings about his personal eligibility for the office,” the International Crisis Group (ICG) think tank said in a note.
Dubbed a political “godfather,” Tinubu campaigned on the slogan “it’s my turn” to govern the country, Africa’s biggest economy.
He has touted his experience as Lagos governor from 1999 to 2007 — a period that, supporters say, modernised Lagos, Nigeria’s commercial hub.
But the new president also faces corruption allegations, which he denies, and questions over his health.
Opposition leaders Atiku Abubakar, who came second, and Peter Obi, who was third, are contesting election results in court, claiming fraud.
The electoral commission acknowledged “glitches” during the vote but dismissed claims that the process was not free and fair.
Buhari had promised to tackle graft and insecurity but disappointed many.
He bequeaths his successor mounting debt and inflation, as well as deepening concern over attacks and kidnappings by armed groups.
His presidency showed “it is possible for an individual believed by many to be personally incorruptible to preside over an administration that is nonetheless defined by corruption and rank incompetence,” said Ebenezer Obadare of the Council on Foreign Relations, a Washington-based think tank.
“Nigerians will soon find out whether a leader widely seen as corrupt can preside over a relatively malfeasance-free and reasonably competent administration,” Obadare wrote in a blog.
Buhari is expected to leave Abuja on Monday to spend time at his farm in Daura, in Katsina State, near the border with Niger.
The two men might be different in style and reputation, but they also have key similarities.
They both adhere to Islam and are of an advanced age, in a country divided between Christians and Muslims and where the median age is 18.
Buhari made repeated medical trips to the UK when he was president, while Tinubu spent time abroad during the campaign and between the election and inauguration.
Speculation about Tinubu’s health is widespread, prompting attention to turn to his deputy Shettima, a 56-year-old former governor of the northern state of Borno.
The new government faces a mountain of urgent work, starting with the economy.
One of the main challenges for oil-rich Nigeria is that it swaps crude worth billions of dollars for gasoline that it then subsidises for its domestic market.
This has caused a huge drain on revenue and foreign exchange, contributing to ballooning debt.
More than 80 million of the country’s estimated 210 million people live below the poverty line, according to the World Bank, and the UN has warned that over of a quarter of those are facing acute hunger this year.
Despite thriving tech and entertainment sectors, many middle-class Nigerians are moving abroad hoping for a brighter future.
Another priority for the incoming government will be to address insecurity, which has spread like wildfire in recent years.
Troops are battling gangs of heavily armed criminals and kidnappers in central and northwestern states, oil thieves, pirates and separatists in the southeast, and a 14-year-old jihadist insurgency in the northeast.
Complicating matters, national assembly elections produced greater political plurality this year, with seven parties represented in the incoming senate and eight in the next house of representatives.
“The next administration will need to work overtime to garner consensus on the legislative agenda,” said Afolabi Adekaiyaoja in a report for the Abuja-based Centre for Democracy and Development.
Governors, who also wield substantial power in Nigeria, were also being sworn in on Monday.