The city has held the number one spot for the past three years because of the high cost of rent, imported goods and security in the oil-rich nation.
However, the rest of the cost of living rankings saw significant shifts due to exchange rate fluctuations.
Asian cities now account for half of the top 10 costliest destinations.
Hong Kong rose to second place and Singapore remained in fourth because of their expensive property and rental markets.
Shanghai, Beijing and Seoul also made the top 10.
Chinese cities now account for nine of the top 30 most expensive cities because of the growing strength of the renminbi.
Tokyo fell out of the top ten for the first time after the yen weakened against the US dollar because of Japan's stimulus program.
It used to top the rankings, but is now in 11th place, down from seventh last year.
N'Djamena, the largest city in Chad, also used to rank consistently highly, but dropped from second to 10th place.
The least expensive cities are Bishkek in Kyrgyzstan, Windhoek in Namibia and Karachi in Pakistan, Mercer said.
The rest were located mostly in Eastern Europe, the Middle East and Africa.
The most expensive European cities are found in Switzerland, with Zurich, Geneva and Bern occupying third, fifth and ninth place in the rankings.
This comes after the Swiss franc jumped following an unexpected move by the country's central bank to remove a ceiling on the currency's value.
But many western European countries saw their cost of living fall due to a slide in the value of the euro.
London remained in 12th spot in the rankings, while Copenhagen slipped nine places to number 24.
The biggest fall from the top 10 was Moscow, which dropped 41 places to 50th because of the depreciation of the rouble following US economic sanctions.
"European currencies have weakened against the US dollar, which pushed most western European cities down in the ranking," said Nathalie Constantin-MÃ©tral, principal at Mercer.
"Additionally, other factors like the eurozone's economy, falling interest rates, and increasing unemployment have impacted these cities".
Across the Atlantic, New York remains the most expensive city in America, followed by Los Angeles, San Francisco and Washington.
Mercer has been conducting the survey for more than two decades using New York and the US dollar as its base city and currency.
The consultancy compares the cost of a basket of over 200 goods and services, including food, transport and accommodation.
The results are then used by governments and multinational companies when determining salary packages for employees being sent overseas.
Several companies, including the Economist Intelligence Unit and ECA International, issue cost of living surveys with differing results.
The Angolan capital, Luanda, remains the world's most expensive city for expatriates, according to an annual survey by consultancy Mercer.