Analysts say the challenge for any incoming government would be to wean the nation away from reliance on oil money and diversify its sources of income into agriculture and manufacturing.
Mr Francisco "Lu Olo" Guterres, a former guerrilla leader from the leftist Fretilin party, was up against seven other candidates.
The energy sector accounted for around 60 percent of GDP in 2014 and more than 90 percent of government revenue.
Lines formed outside polling booths, as voters queued to choose from a field of eight candidates contesting the fourth presidential poll since East Timor won independence from Indonesia in 2002. His chances were further enhanced by the endorsement of resistance hero Xanana Gusmao and his CNRT party, said Michael Leach of Australia's Swinburne University.
"If I am later chosen to be president of East Timor, I will prioritise the economic and education sectors, to support the welfare of the people", Guterres said while campaigning.
The new government will inherit a looming budget crunch as its main source of revenue, the Bayu-Udan field, operated by ConocoPhillips, is set to dry up in the next five years.
Guterres' Fretilin party began as a resistance movement against Portuguese and then Indonesian rule.
Many of its key figures still feature prominently in the running of the country.
Monday's election is the first since UN peacekeepers left the half-island nation in 2012.
His closest rival, Antonio da Conceicao, is in second position with 32 per cent. The education minister has secured backing from his own party as well as the newly formed People's Liberation Party of the incumbent president Jose Maria de Vasconcelos.
"He has good plans that I believe can take this country to a better future", she said.
While the president's role is largely ceremonial, it is also seen as important for promoting unity in the young nation.
Mr Ruak was not up for re-election and is expected to make a run for prime minister in July's parliamentary elections.
To avoid a second round, a candidate has to win more than 50 per cent of the vote.
Preliminary results are expected to be known within a few days.
- Barcelona's Neymar eyes Premier League switch in the future
- CCTV Of Woman's Terrifying Close Call With Sexual Predator
- Neuer withdraws from Germany squad
- Firing by SSB: Tension along Nepal border
- Schumer: Trump must apologize for wiretapping claim
- Desaparece policía federal en Irapuato
- Scotiabank Reaffirms Outperform Rating for Silver Wheaton Corp. (SLW)
- Expresa AMLO respeto a Ejército
- German Defense Ministry says "no debt at NATO" after Trump's tweets
- Colin Farrell to star as Oliver North in Amazon miniseries