The Milwaukee County Transit System (MCTS) is the largest transit agency in Wisconsin, and is the primary transit provider for Milwaukee County. The agency ranks among the top 50 transit agencies in the United States for total passenger trips. The agency is partially managed by the quasi-governmental agency Milwaukee Transport Services, Inc. Its bus fleet consists of 369 buses. In 2025, the system had a ridership of 25,328,400, or about 73,200 per weekday as of the fourth quarter of 2025.

Public transit operations began in Milwaukee in 1860. The service consisted of two horse-drawn cars. On June 1, 1975, Milwaukee County took over the bus system and established the Milwaukee County Transit System after taking over the assets of the Milwaukee & Suburban Transport Company, a private operator.

In 2009, Wisconsin Governor Jim Doyle proposed a three-county Regional Transit Authority that would incorporate MCTS. The proposal faced opposition from some lawmakers and the Regional Transit Authority was never created.

In late 2013 and into the early part of 2014, MCTS began debuting new technology on board buses. This included introducing real-time bus information allowing passengers to track the exact location of buses, new fareboxes and an electronic fare system by virtue of a smart card (M•CARD), and a stop announcement system complete with visual and audio information. Clever Devices is the provider of the real-time bus information and stop annunciator system, whereas the new fareboxes were provided by Scheidt & Bachmann.

In 2018, MCTS began a comprehensive study and overview of the entire fixed-route system and began the process of implementing a new system with faster service, more connections, and easier-to-understand routes. Multiple community meetings and forums were held to gather public input on the project. Before the project, only about 40% of the system was high-frequency routes, defined as routes on which buses come every 15 minutes during peak hours. The consensus from the study was that riders wanted faster service and were willing to walk an extra distance to bus stops. MCTS presented two options - transition the system to an 80-20 model, meaning 80% of the routes would be high-frequency, or a 60-40 model with 60% of the routes high-frequency, the latter being what the public ultimately decided. MCTS analyzed every bus stop in the system and removed some lightly used bus stops to speed up service. The system overhaul was implemented in three phases during 2021, starting on March 7. This first phase included modifications to several routes and introduced two new ones. The second phase, on June 6, 2021, involved changes to the PurpleLine and various numbered routes, plus four new additions. The final phase on August 29, 2021, affected the BlueLine and multiple other routes, added three new ones, and retired two lines. Ridership increased by 14% after the first phase was implemented.

Due to the COVID-19 pandemic, face masks were required on all MCTS buses. In the early stages of the pandemic, there was a passenger limit of 10 passengers per bus, which was increased to 15, but rescinded on July 1, 2021. Passengers were encouraged to limit interaction with the bus driver, exit through the back door, and to use contactless fare forms, such as the M•CARD or Ride MCTS app.

In mid-2023, MCTS transitioned from the M•CARD pass scheme to the WisGo smartcard and Umo mobile app. The M•CARD became invalid on October 1, 2023. A non-expiring stored value system replaced fixed-period passes, and the previous Go Pass and reduced fare programs were merged. Tap-to-pay validators replaced the prior method of presenting codes to transit personnel for mobile app users.

As of 2026, the system supports digital payments through various credit card and mobile payment methods. WisGo and Umo users receive free time-based transfers. Fare capping is also a core feature, limiting fares to set daily, weekly, and monthly maximums. On-board cash payments remain an option, but do not qualify for transfers or fare capping.