Lee will replace retiring Matthew Cheung Kin-chung as chief secretary.
The Police Commissioner Chris Tang Ping-keung will take over as the secretary for security.
The appointments were announced by Beijing as a part of a government reshuffle.
Raymond Siu Chak-yee, one of Tang’s deputies, will be the new commissioner of police.
Speaking at a press conference today, Lam expresses her sincere gratitude to the out-going Chief Secretary Cheung.
“Cheung had served Hong Kong for over half a century and had made major contributions,” said Lam.
Despite only having a year left of her current term, Lam said “now is a good time to step forward and start anew".
She also thanked Beijing for its trust in her and its support during two "challenging" years, saying under the central government’s leadership, the SAR has emerged from chaos.
Lam said the promotions were made on merit, urging the society not to make unnecessary speculation.
She said the three officials “are the best candidates at this moment”.
When asked about Lee lacking experience in other policy bureaus, Lam said the full range of experience is not one of the most important factors.
"To be fair to Lee, he has been in the government for almost four decades. And the Security Bureau is perhaps one of the biggest, in terms of significance, in terms of the number of civil servants under its supervision. So one could not say that as the secretary for security for many years, his experience is limited to the police background or limited to the law enforcement," she said.
Commenting on his appointment, Lee said he is proud to be appointed by the Central Government as the next Chief Secretary, and will render his full support to the Chief Executive in administering Hong Kong according to the law.
It will be the first time a former police officer from the security branch of the government will take up a top role in the administration.
Lee was deputy police commissioner in 2010 before he was appointed to deputy head of the Security Bureau in 2012. He was promoted to the bureau’s top position in 2017.
He said he will proactively coordinate work across government bureaus and departments.
“I will facilitate the Legislative Council’s role in monitoring the government and also implement the political ideal of patriots ruling Hong Kong...supporting the chief executive in the fight against the epidemic and push for economic development,” Lee added.
While Tang said he will work on immigration, import-export control, prisoners rehabilitation, anti-terrorism and national security being the city’s security chief.
Mentioning the upcoming elections, Tang said he will “ensure that these take place under a safe and stable environment.”
Beijing’s imposition of a sweeping national security law in Hong Kong in June 2020 was China’s first major step to set the global financial hub onto an authoritarian path, impacting most aspects of life in the city.
Lee, Tang and Secretary for Justice Teresa Cheng Yeuk-wah have been key local players in Beijing’s push to reassert its control over Hong Kong.
Lam and about 60 senior public figures are set to travel to Beijing on Monday for the Communist Party’s centenary celebrations, the first time the chief executive will be out of town for the official anniversary of Hong Kong’s 1997 handover.