As he seeks re-election, here is a look back at key developments in the country during his first term.
2015: successes against Boko Haram
Two months after he is sworn in, Buhari in July 2015 sacks the military top brass he inherited from his predecessor Goodluck Jonathan, and galvanises regional support against the jihadists.
A succession of announcements are made that hostages have been rescued and the rebels pushed back.
In November a new government is finally sworn in, more than five months after Buhari took office.
In December the president says Boko Haram is “technically defeated” and “no longer capable” of conventional attacks, though suicide bombers remain a threat.
In May 2016 local militants attack an oil installation that belongs to US energy group Chevron. It is the first of a series of acts of sabotage which will slash oil production by one million barrels a day over the year.
After negative growth in the first two quarters, the economy enters recession for the first time in 25 years, burdened by attacks on oil facilities, a fall in crude prices, rampant inflation and historically low foreign investment.
In August, the Islamic State group recognises a Boko Haram faction that broke away from long-time leader Abubakar Shekau in opposition to his indiscriminate targeting of civilians.
In October, after negotiations with the government, Boko Haram frees 21 of the 276 schoolgirls kidnapped from Chibok in Borno state in 2014, an incident that drew world attention.
2017: mounting health concerns
In January 2017, 112 displaced people are killed when an air force jet accidentally bombs a camp in northeastern Nigeria instead of Boko Haram militants.
In May, 82 more Chibok schoolgirls are released after more than three years in captivity.
Buhari spends more than 100 days in London for medical treatment for an undisclosed illness, returning home in August.
He had already spent nearly two months in London in January and February. Questions mount about his fitness to hold office.
In October the government agrees to compensation for victims of the 1967-1970 civil war in southeastern Biafra, paying out 50 billion naira (139 million euros).
2018: rise of IS-backed Boko Haram
In February, fighters from the self-styled Islamic State West Africa Province faction of Boko Haram abduct 110 young girls from a school in Dapchi in Yobe state. A month later most are freed.
In June clashes between nomadic cattle herders and indigenous farming communities in central Nigeria leave more than 200 dead, the latest in a long-running battle for land and resources.
Calls are made for Buhari to restore order or resign, with anger also over his apparent one-sided fight against corruption, which has mostly targeted political opponents.
In July Buhari’s All Progressives Congress party suffers a wave of defections to the main opposition Peoples Democratic Party.
(ISWAP) steps up its deadly attacks on military bases, killing scores of soldiers and capturing weapons and hardware.
2019: top judge suspended
In January 2019 the opposition objects to the appointment of Buhari’s niece to the commission organising the February elections, which dismisses their concerns.
Later that month Buhari suspends the head of the Supreme Court after he is charged with breaching rules for public officials about disclosure of assets.
Lawyers, lawmakers and the opposition accuse Buhari of interfering in the judiciary, amid repeated claims the ruling party is trying to rig the vote.