Professor Thomas specializes in the field of international political economy. His main research interests are capital mobility, multinational corporations, and controlling competition for investment. His first book, Capital Beyond Borders: States and Firms in the Auto Industry, 1960-1994 (St. Martin's), was published in 1997, and his second, Competing for Capital: Europe and North America in a Global Era (Georgetown), was published in 2000. The latter book included the first-ever estimate of total state and local subsidies to business in the United States.

In 2007, he was the Canada-U.S. Fulbright Scholar in North American Politics and Society at Carleton University, studying Canadian efforts to keep provinces from using subsidies to induce the relocation of investment.

His third book, Investment Incentives and the Global Competition for Capital, was published by Palgrave Macmillan in December 2010. This book includes a new estimate for state and local investment incentives and subsidies.

Professor Thomas has consulted for the International Institute for Sustainable Development. In November 2007, IISD's Global Subsidies Initiative published Investment Incentives: Growing Use, Uncertain Benefits, Uneven Controls. In 2008, he worked with research teams from Vietnam, Indonesia, Singapore, and Malawi for an IISD project on investment incentives and sustainable development. In November 2009, he represented IISD at a workshop on investment incentives organized by the World Association of Investment Promotion Agencies in Ljubljana, Slovenia.

Professor Thomas has not neglected the state and local aspects of competition for investment. He has been active on the issue of tax increment financing (TIF) in Missouri since 1998. His most recent research on the topic is Susan Mason and Kenneth P. Thomas, "Exploring Patterns of Tax Increment Financing Use and Structural Explanations in Missouri’s Major Metropolitan Regions” appeared in the July 2018 edition of the HUD journal Cityscape.