The 1950s brought even greater growth and influence for the Municipal League. Its roster swelled to 5,206 members in 1952 (though a dues increase, trimmed them back to almost 4,000 by the end of the decade). With this strength, the League undertook one of its greatest achievements, the creation of the Municipality of Metropolitan Seattle, or '“Metro.”

A young attorney named James R. Ellis called for a "metropolitan government" for the region in a 1953 speech. Scientists were predicting that Lake Washington had only a few years “to live" before it choked on algae fed by the raw sewage pumped into it by neighboring cities. The proposal to form Metro was formally embraced by the League in its 1955 report, “Metropolitan Seattle-The Shape We're In." Ellis was one of the principal authors. The League went on to help pass a state law in 1957 to permit the creation of a county-wide "municipality" for water quality, waste management, transit, and parks supervision. With legislative approval, the initial charter for Metro went to the voters in 1958 and succeeded on its second try. This launched the initial $135 million program to clean up Lake Washington.

The League was exceedingly active during the 1950s. It campaigned hard but unsuccessfully for a new home rule County Charter in 1952; championed creation of the City Transit Commission; led the fight to expand the Port Commission from three to five elected members; secured the election of municipal and traffic judges; prodded the Seattle School District into reforming its civics and social studies curriculum; arranged for the first evaluation of municipal finances and operations by an independent private accounting firm; drafted Seattle's first noise control ordinance; and promoted the installation of sprinklers in the city's older, wood-framed schools.

The League maintained this level of activity into the 1960s. Again, it was James Ellis who laid down the League's most important challenge when, in 1966, he delivered a speech entitled "The City-A Cause Waiting for Rebellion." He challenged the community to take control of its own accelerating growth through a comprehensive program of capital improvements, including rapid transit and new parks, roads, and civic facilities. The proposal was soon augmented with additional ideas, such as that for a "county domed stadium," and became known as Forward Thrust." Forward Thrust bonds for a total of $334 million were passed by the voters in 1968 for virtually all of the projects, with the notable exception of rapid transit.

(In 1960, Jim Ellis was rewarded with the League's first "Outstanding Citizen" Civic Award for his accomplishments. He won it again in 1968 for his leadership of Forward Thrust. In 1989, the League, gave Ellis his own Civic Award: the James R. Ellis Regional leadership Award, to commemorate a lifetime of civic achievement.)

In 1968, the League finally won its 2-year battle to create a home rule Charter for King County. This created the nine member elected County Council and the County Executive seat, and it allowed department heads to be appointed rather than elected. The decade ended in a flurry of activity on behalf of a County Ombudsrnan's Office; a County hearing examiner system; a household tax to help subsidize Seattle Transit; and, as a token, of the' "good old days," a grand jury investigation into vice, graft, and payoffs in the Seattle Police Department

Transportation problems headed the Municipal League's agenda, for most of the 1970s, beginning with its research into proposals for the I-90 bridge and a proposed Bay Freeway between I-5 and Seattle Center. The League scored major victory in 1972 when the voters delegated management of a county-wide transit system to Metro. Its most impressive accomplishment of the decade, however, was the revelation of massive irregularities in the design and budgeting of the proposed high-level West Seattle Bridge. The League's first findings spurred a grand jury investigation that culminated in the conviction of the city engineer and two powerful State legislators for corruption.

In 1972, ever vigilant of good government, the League joined with many civic organizations to promote Initiative 276, which created the Public Disclosure Commission, open meeting laws, and the reporting of campaign contributions. Also that year, the League challenged a city proposal for $40 million in bonds to repair bridges, and when voters rejected the bonds, it helped the city to find $8 million in unspent funds to do the job. It also investigated irregularities in Seattle's attempt to acquire a computerized financial system, which led to sweeping reforms in city purchasing proposals.

In 1973, the League advocated election of Freeholders to draft a new City Charter, but the panel split over whether to create City Council districts (an issue that had also divided the League in 1911), and the proposed Charter failed. The League rebounded in 1977, helping to win passage of four out of five proposed amendments to update the 1946 Charter it originally helped to write. As the decade closed, the League lent its support to the Seattle School District's desegregation plan and it promoted revision of Seattle's Comprehensive Plan.