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 power project, which began supplying power to Seattle 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.

Construction of Ruby Dam, Steps One and Two, began in 1937 after City Light received three million dollars from the federal Public Works Administration to begin clearing timber from Ruby Basin. The dam was the third built by Seattle City Light as part of the Skagit River hydroelectric power project. It was renamed Ross Dam in 1939 in memory of James D. Ross, the long-time superintendent of City Light who died in March of that year. The first step of Ross Dam's construction was completed in 1940. The dam was 305 feet high and created a reservoir with a water level at 1380 feet above sea level. Ross Dam was initially constructed for storage rather than power generation.

Step Two in the construction of Ross Dam began in 1943. The dam height was to be raised to nearly 500 feet. Step Three was approved by the Federal Power Commission (FPC) in 1947 and at completion in 1949, the dam stood at 540 feet. Ross Lake, the reservoir behind the dam, rose to 1615 feet above sea level, well above the elevation of the Skagit River at the British Columbia border.

In 1950, the FPC authorized construction of Ross Powerhouse and three generating units. During 1952 to 1956, Ross Powerhouse was constructed and four generators went on-line, doubling the electrical output generated by City Light. The fourth generating unit was installed at Ross Powerhouse in 1956.

The United States Congress created Ross Lake National Recreation Area and the North Cascades National Park in 1968. In 1970, Seattle applied to the FPC for construction of a fourth step of Ross Dam, which would raise the reservoir elevation to 1725 feet. The final water level of Ross Reservoir was not settled until 1984, after lengthy negotiations with the International Joint Commission comprised of representatives from the State of Washington, City of Seattle, and British Columbia, Canada. A fourth step was authorized in 1977, but authorization was revoked in 1984 by an 80-year agreement between Seattle and British Columbia stipulating that the fourth step would not be constructed, that British Columbia would supply power to Seattle in lieu of construction, and that Seattle would pay British Columbia an amount relative to construction costs. This agreement also created the Skagit Environmental Endowment Commission, a bi-national agency who mission is to foster education about the upper Skagit River watershed.