The deals aim to provide Hong Kong with a consistent supply of both natural gas and nuclear energy for the next two decades, as it tries to reduce its dependency on coal-fired power, chief executive Donald Tsang said.
"The sustained supply of clean energy from the mainland will greatly reduce the need for Hong Kong to build a liquefied natural gas (LNG) terminal within its territory," he told reporters.
"The resulting reduction in capital investment by the power company concerned will relieve the pressure for electricity tariff increases."
"(Hong Kong) can also benefit from improved air quality by increasing the use of clean energy and reducing the emissions of power plants."
Hong Kong would sign a 20-year natural gas supply agreement with China's third largest oil company, CNOOC Group, and a 20-year nuclear power deal with neighbouring Guangdong province, a government statement said.
There will also be a feasibility study for supplying further natural gas to Hong Kong via a pipeline from central Asia and the deal meant Hong Kong firms could build a gas terminal in mainland China.
Tsang signed the agreement with Zhang Guobao, China's top energy official, in Hong Kong. The memorandum of understanding guarantees there will be no reduction in supply from current levels.
Hong Kong relies on coal-fired power for much of its energy, which is one of the major contributors to pollution from within the territory.
The city has faced criticism that it is not doing enough to reduce the smog, much of which originates across the border and has raised worries it could damage the economic centre's ability to attract top international talent.
Local energy firm China Light and Power has been pushing to build an LNG terminal on one of Hong Kong's islands, but has hit concerns from environmental groups that the proposed location could damage the local wildlife.