"(Buhari) does not consider the 100 days in office as a milestone... There is no official event to mark the day," Garba Shehu told AFP.
Buhari ran as a hawk on security and with a tough anti-corruption stance, pledging to recover "mind-boggling" sums of stolen oil money and vowing to crush Boko Haram's six-year Islamist insurgency that has killed at least 15,000.
The 72-year-old president is determined to "fix insecurity, the economy and eliminate corruption. These are the goals he has been pursuing in the past 100 days," said Shehu.
The leader has accused former president Goodluck Jonathan, who ruled from 2010 until May, of having left the treasury "virtually empty."
Buhari's vice-president, Yemi Osinbajo, has estimated the country's debts at some $60 billion (54 billion euros), and said Nigeria's economy is in its worst state since the country gained independence 55 years ago.
Local media have been largely supportive of Buhari thus far, comparing him favourably with Jonathan.
But Buhari's own slowness in forming a cabinet is hurting Africa's largest economy, said Paul Igbinoba, a Lagos-based economist and political commentator.
"The economy has suffered from the delay... The president needs to move quickly in appointing his key ministers and putting in place his economic team," he said.
Yet there have been tangible improvements under his rule.
Electricity production rose from less than 3,000 megawatts before Buhari came into office to about 5,000 megawatts now, while refineries have suddenly come back to life after years of idleness.
In an early sign of his assault on graft, Buhari sacked the entire board of state oil company NNPC, notorious for mismanagement and rampant theft. He then installed a Harvard-educated lawyer to spearhead reforms as the new managing director.
The military has also intensified its war against Boko Haram in the northeast, using its air power to support ground troops.
Buhari recently set a three-month deadline to subdue the insurgents.