After graduating college, he was named Assistant in Charge of Good Roads Investigation for the Iowa State Highway Commission (ISHC). He then became chief engineer and then Iowa highway commissioner, overseeing a budget of just $5,000 a year. He was soon named President of the
American Association of State Highway Officials (AASHO) and at the age of 38, was suggested by that group to serve as chief of the Bureau of Public Roads. Congress quickly accepted.
He demanded (and received) a salary increase from $4,500 to $6,000 and remained an AASHO board member. He also insisted on adoption of "the most liberal policy possible under the existing laws, in order to get actual construction work under way as early and as rapidly [as reasonable]."
MacDonald quickly pulled together a coalition including the Portland Cement Association, the American Automobile Association, the American Road Builders Association, the Association of Highway Officials of the North Atlantic States, the Rubber Association of America, the Mississippi Valley Association of State Highway Officials, the National Paving Brick Manufacturer's Association, the National Automobile Chamber of Commerce, and more.