
Metro was reluctant to adopt low-floor buses, not buying any until 2003. Metro's entire fleet has been wheelchair-accessible since 1999. Early lifts were severely flawed, but by the mid-1980s the lifts were generally reliable and were ordered on all new buses. In 1979, the agency ordered some of the first wheelchair lift equipped coaches in the nation, promising a completely new level of independence for disabled residents.

Today, King County Metro has one of the largest articulated fleets in North America (second only to MTA New York City Transit) and articulated buses account for about 42% of the agency's fleet. In 1978, Metro was the first large transit agency to order high-capacity articulated buses (buses with a rotating joint). The fleet of 151 buses were manufactured by German maker MAN as part of a bulk order with other large U.S.

In 1978, Metro became the first large transit agency in North America to introduce articulated buses to its fleet, which required some bus stops to be rebuilt to accommodate 60-foot (18 m) coaches. The first fleet of new 40-foot (12 m) buses ordered by Metro arrived in June 1976, consisting of 145 diesel coaches manufactured by AM General.

Metro acquired the 91-bus fleet of the Metropolitan Transit Company in December 1972 at a cost of $2.75 million. Upon taking over transit operations on January 1, 1973, Metro used buses acquired from predecessor agencies Seattle Transit System and the Metropolitan Transit Company, still painted in their original colors. As of 2017, King County Metro operates the 10th largest fleet of buses in the United States, with a total of 1,540 buses.
