Donald Tsang took his oath of office in the Great Hall of the People, the seat of China's legislature, where he met Prime Minister Wen Jiabao.
Tsang replaced Tung Chee-hwa, who resigned in March citing failing health.
Many believe that Tung was pushed out by Beijing, which had lost confidence in him. Tung suffered from dismal public approval ratings during his eight years in office.
Wen heaped praise on Tsang, saying the veteran civil servant would "unite all the people of Hong Kong, and have great achievements in making Hong Kong more prosperous and stable".
Tsang told Wen: "I'll definitely continue to work hard on the things the premier wants me to do for Hong Kong's future. I'll do everything that I can do to do what the country wants me to do for the well being of the Hong Kong people."
Tsang faces two major challenges: Keeping the economic recovery on track and pleasing Beijing, which has rejected the public's calls for full democracy.
Tsang, a policeman's son who did not go to university, has spent 38 years in the Hong Kong government.
"I'll definitely continue to work hard on the things the premier wants me to do for Hong Kong's future. I'll do everything that I can do to do what the country wants me to do for the well being of the Hong Kong people"
Hong Kong chief executive
He said on Tuesday that he would listen closely to the public and that he had already gone into the "streets and alleyways" to hear the people's stories.
Tsang overwhelmingly won the backing of an 800-member committee that selects Hong Kong's leader, or chief executive.
Hongkongers were never allowed to directly elect their leaders during British rule, and the undemocratic tradition has continued under China's governance.
Massive street protests have been held to demand direct elections, but China has ruled that Hong Kong is not ready for full democracy.
Tsang sides with Beijing on the issue, but said on Tuesday that a task force was considering political reforms for the 2007 leadership election.
Another protest was scheduled for 1 July, but Tsang said he had no plans to participate.
During the past eight years, Hong Kong's economy has been hammered by the Asian financial crisis in 1997-1998 and an outbreak of severe acute respiratory syndrome, or Sars, in 2003.
But the territory has recently rebounded, with the economy expanding 6% in the first quarter of 2005.