The West African country, which never fully recovered economically from a 1991-2002 civil war and the Ebola epidemic a decade later, was further pummeled by the COVID-19 pandemic and fallout from the conflict in Ukraine.
The two will face off for the second time in a row after Bio, of the Sierra Leone People's Party (SLPP), narrowly beat Kamara in a runoff in 2018.
Rising food prices are a key issue for many voters in the import-dependent West African nation of eight million people. Year-on-year inflation hit 43% in April, according to the latest official figures.
Both Bio and Kamara told AFP they would prioritise boosting agricultural production.
Some 3.4 million people are registered to vote, 52.4% of whom are under 35 years old, according to an electoral commission spokesman. Polling opens at 7:00 am and closes at 5:00 pm (17:00 GMT).
Presidential candidates must secure 55% of valid votes for a first-round win. Turnout has ranged between 76 and 87% over the past three elections.
Voters will also elect members of parliament and local councils in a proportional representation system after a last-minute switch from a first-past-the-post system. Under a recently passed gender act, one-third of all candidates must be women.
A new 11.9% vote threshold will make it difficult for independents and minority parties to secure seats in parliament.
Many Sierra Leoneans vote based on regional allegiances. The majority of people in the south and east normally vote for the ruling SLPP. The majority of people from the north and west normally vote for the opposition APC. Jobs and benefits are commonly perceived to flow to regions whose politicians are in power.
Kamara is facing a protracted trial over allegations that he misappropriated public funds as foreign minister, a case he says is politically motivated.
A June 14 poll by Institute for Governance Reform (IGR), a partner of the pan-African survey group Afrobarometer, forecasts Bio will win 56% of the vote, with 43% for Kamara.
Another poll, conducted by the newspaper Sierra Eye and two local data groups, forecasts 38% for the incumbent and 25% for his main challenger.
Both candidates on Thursday evening encouraged their supporters to vote peacefully and responsibly.
The elections are being closely followed in West Africa, a region recently dominated by coups and turmoil. A group of foreign ambassadors on Wednesday issued a joint statement calling for peace following reports of election-related "aggression".
Security forces clashed with APC supporters Wednesday in the capital Freetown. Amnesty International and Reporters Without Borders have called on authorities to protect freedom of expression and press freedom during the election.
Last August, riots left at least 27 civilians and six police officers dead. Online disinformation campaigns have contributed to the violence.