Frank G. Moran established the Moran-Lakeside School in 1919 as an independent preparatory school for boys in Seattle's Denny-Blaine neighborhood, on the shore of Lake Washington. The school was designed as a feeder for Moran's other institution on Bainbridge Island. A group of parents incorporated the school in 1923 and renamed it Lakeside Day School, and the following year it moved to the present site of The Bush School in Washington Park. The school eventually settled on its current north Seattle campus, where three new buildings completed a merger that consolidated its scattered operations. What began as one man's educational ambition became, over the decades, something more self-sustaining: a school shaped less by its founder's vision than by the families and teachers who kept reinventing it.

Lakeside's most consequential decision may have been that teletype purchase. When Gates and Allen gained access to the terminal in 1968, they dove into programming with an intensity that alarmed and fascinated their teachers. Allen, class of 1971, and Gates, class of 1973, spent every available hour at the machine, eventually exhausting the school's computing budget and negotiating time on other systems. Their classmate Ric Weiland, also class of 1971, joined them; he would become a pioneering software engineer and one of the most generous LGBT philanthropists in American history. The school did not set out to incubate a technology revolution. It simply made a resource available and got out of the way, a philosophy that extended to its broader curriculum: no Advanced Placement courses, no International Baccalaureate program, just faculty-designed classes and a requirement that students complete eighty hours of community service and a week-long outdoor program.

Lakeside's roster of graduates spans an improbable range. Adam West, class of 1946, became the original Batman on television. Booth Gardner, class of 1954, served as governor of Washington and chaired the National Governors Association. Edward Ferry, class of 1959, won an Olympic gold medal in the coxed pair at the 1964 Tokyo Games. Craig McCaw, class of 1968, founded McCaw Cellular, which became a cornerstone of the modern wireless industry. Frederic Moll, class of 1969, co-founded Intuitive Surgical and helped pioneer robotic surgery. More recently, Corbin Carroll, class of 2019, became the 2023 National League Rookie of the Year for the Arizona Diamondbacks. A school of nine hundred students producing this density of achievement across such different fields is not easily explained by resources alone, though the school's endowment, $260 million as of 2024, certainly helps.

In 1965, Lakeside launched the Lakeside Educational Enrichment Program, known as LEEP, a free summer program that enrolled the school's first Black students and became one of the longest continuously running educational enrichment programs in the country. The program marked the beginning of a deliberate, if gradual, effort to diversify a school that had started as an all-boys institution for Seattle's privileged families. Today Lakeside offers need-based financial aid to families earning under $250,000 and maintains affinity groups including the Black Student Union, GLOW (a gay-straight alliance), and organizations supporting Asian, Pacific Islander, Latin American, and multiracial students. Over eighty official clubs and a newspaper called the Tatler fill the extracurricular calendar. Whether a school this expensive can truly be called accessible remains a fair question, but the effort to open its doors wider than its founders imagined has been sustained for six decades and counting.