In 36 years as a hard-driving, innovative administrator with the Michigan Department of Transportation (1948-1984), McCarthy played a lead role in the design and construction of Michigan's freeway system. As the youngest district engineer in the Department's history, he supervised projects in east-central Michigan, then became chief of highway design and traffic statewide. He retired as deputy director for highways when the state's interstate freeway system was 90 percent complete, helping to put Michigan at the forefront in building its 1,241-mile share of the national network. He also fostered such cost-saving and pace-setting efforts as the Department's national computer-automated system for designing highways and bridges.