The Tamil National Alliance (TNA) swept all five districts in the Northern Provincial Council which went to the polls Saturday, the department of Elections results showed.
The TNA bagged 30 out of a total of 38 seats in an election held under a system of proportional representation.
In the most populous district of Jaffna, the TNA secured more than 84 per cent of the popular vote, exceeding the party's own projections of 66 per cent.
President Mahinda Rajapakse's United People's Freedom Alliance was a distant second with just seven seats while a Muslim party picked the other seat.
The TNA, which has vowed to press a demand for "self-rule" for Sri Lanka's ethnic Tamils in the majority Sinhalese nation of 20 million, made a clean sweep in the war-affected region despite allegations that the army was trying to discourage TNA voters.
The government said 68 per cent of the 719,000 electorate in the Northern Province turned out to vote Saturday in an election the UN had seen as an important for ethnic reconciliation in a country where over 100,000 have been killed in decades of ethnic violence.
The Sinhalese-dominated government called the election under international pressure to share political power with the Tamils four years after defeating separatist Tamil rebels who had fought for full independence.
The TNA, a coalition that includes moderate Tamils, former Tiger militants and their opponents, has charged that the military was intimidating supporters and forcing them to stay away from voting.
At one time, the TNA was regarded proxies of the separatist Liberation Tigers of Tamil Eelam (LTTE), who were crushed by government forces in a major offensive that ended in May 2009.
The LTTE had led a violent campaign for full independence from 1972 till they were defeated four years ago.
Election officials said they had "plenty of complaints" before and during Saturday's vote, but there were no major incidents during a nine-hour voting period on Saturday.
Saturday's poll is also the first local election to be internationally monitored. Dozens of observers from South Asian nations and the Commonwealth were involved along with hundreds of private monitors.
The Northern Provincial Council was set up in 1987, but elections were never held and its functioning was controlled directly by the Sri Lankan president because of fighting in the region.
Retired Supreme Court judge Kanagasabapathy Wigneswaran is now expected to be the region's first elected chief minister in a council that will have limited powers over the local administration.
Wigneswaran, who turns 74 next month, has said he wants to work with Colombo on pushing his manifesto, which calls for "self-government" for Tamils.
Wigneswaran's priorities are payment of war reparations, securing an army pull-out from the former combat zone and taking back land the military still occupies four years after defeating Tamil Tigers who fought for full independence.
President Rajapakse has accused the TNA of raising expectations of a separate state that is opposed by the majority Sinhalese.
The latest results are a setback to Rajapakse has won almost every major election since he led the campaign that crushed Tamil Tigers in 2009.
The spectacular military success triggered international calls to probe allegations his troops killed up to 40,000 Tamil civilians in the final months of fighting.