It was the fourth victory for the Democrats after nine years of running City Hall.
"I would like to thank you all for giving me another chance and for trusting the Democrat Party to run the capital for 12 years. The party will go ahead to make it be 16 years. I promise that I will work harder for all, even for those who did not vote for me," Mr Sukhumbhand told a cheering crowd of supporters at the party headquarters.
"I would like to thank the prime minister for pledging to work seamlessly with me and I will also do likewise," he said.
The former governor celebrated his re-election victory shortly after his rival from the ruling Pheu Thai Party Pol Gen Pongsapat Pongcharoen conceded defeat after more than 80 per cent of the unofficial tally showed Mr Sukhumbhand leading with more than one million votes with the former deputy police chief receiving more than 800,000 votes.
An unofficial result as of 8.08pm when 100 per cent of the vote tally was complete, Mr Sukhumbhand won 1,256,231 votes, followed by Gen Pongsapat with 1,077,899 votes, and former police chief Seripisut Temiyavej garnering 166,582 votes followed by 78,825 votes for Suharit Siamwalla, an independent candidate.
Voter turnout overall accounts for 2,715,640 people or 63.98 per cent of the total 4,244,465 eligible voters.
Gen Pongsapat earlier congratulated his Democrat rival for securing the City Hall office for a second term, promising to work with Sukhumbhand for a better life for Bangkokians.
Speaking at a joint news conference at Pheu Thai party headquarters, Ms Yingluck gave congratulations for the unofficial victory of Mr Sukhumbhand, pledging that her government is ready to work with the newly-reelected governor smoothly in the future.
The premier thanked all voters who supported for the Pheu Thai candidate and assured Bangkokians that her party will continue to serve them after learning their grievances while on the campaign trail.
The Thai capital of Bangkok is a special administrative zone. Its governor is elected directly by eligible voters and has a four-year term in office.