Seattle City Light provides electricity and electrical and conservation services to its public and private customers. It is the largest public utility in the Pacific Northwest. Public responsibility for electrical energy dates to 1890 with creation of the Department of Lighting and Water Works. In 1902, Seattle voters passed a bond issue to develop hydroelectric power on the Cedar River under the administration of the Water Department. Electricity from this development began to serve Seattle in 1905. A City Charter amendment in 1910 created the Lighting Department. Under the leadership of Superintendent James D. Ross, the department developed the Skagit River hydroelectric project, which began supplying power in 1924. Both public and private power were supplied to Seattle until 1951 when the City purchased the private electrical power supply operations, making the Lighting Department the sole supplier. The Boundary Project in northeastern Washington began operations in 1967 and supplied over half of City Light's power generation. By the early 21st century, approximately ten percent of City Light's income came from the sale of surplus energy to customers in the Northwest and Southwest with the remainder of City Light's financial support coming from customer revenue. The current name of the agency was adopted in 1978 when the Department was reorganized.

In 1917, Seattle City Light -- headed by Superintendent James D. Ross -- applied for permission to develop the Skagit River for hydroelectric power. In 1918, City Light received permission from the federal government to construct the Gorge and Newhalem powerhouses; the first task was to construct a 31-mile railroad for the purpose of carrying workers and equipment to the construction sites. Newhalem Dam was the first to be constructed, and Newhalem Powerhouse went live in 1921, supplying power not to Seattle but for the construction effort. Due to the rising cost of the project, the Gorge Dam began as a wooden one, later to be replaced by a concrete structure. Power from the Gorge Dam reached Seattle in September of 1924.

Over the years, three dams would be constructed along the Skagit River. The first of these, completed in 1930, was Diablo Dam. At 389 feet, it was at the time the tallest dam in the world. In 1937, construction began on Ruby Dam, which was renamed Ross Dam after James D. Ross' death in 1939. This dam was originally intended to be used for storage rather than generation. The reservoir created by Ross Dam eventually flooded into British Columbia. After a series of negotiations, Seattle came to a 1984 agreement with British Columbia that no further construction would occur on Ross Dam; instead, Seattle would purchase power from British Columbia. The Gorge High Dam was completed in 1961.

Today, these three dams supply twenty-five percent of Seattle's power, and Skagit Tours, which began as a tourist attraction in the 1920s, continues to be popular.