Thomas was appointed Honorary Professor at University of Copenhagen in 2009 for a period of 5 years. This position was extended in 2014 until 2019. In 2013- 2015 he was acting as Professor at SLU in a 20% position, and became a full professor at SLU in Urban Open Space Management in June 2016.

Thomas was co-founder (with Cecil Konijnendijk) of the scientific journal Urban Forestry and Urban Greening , published by Elsevier since 2002. During his career, Thomas has produced more than 350 publications, including peer-reviewed articles and book chapters, edited proceedings and books, scientific and professional articles, and conference papers. He serves as associate editor for three journals, and reviewer for a number of peer-reviewed publications related to green space planning and management.

Thomas has been involved in a range of international networking and project activities. He initiated and coordinated the COST Action on Urban Forests & Trees (funded by the European Union), and participated in the twinning cooperation for sustainable forestry between Denmark and Malaysia. He has advised the Food and Agriculture Organization of the United Nations on urban and peri-urban forestry issues, and has served as project evaluator in e.g. USA, UK, Singapore and Hong Kong.

Thomas has served on a number of boards, including the Danish Association of Landscape Architects (1997-1999), International Society of Arboriculture (2000-2005), Danish Tree Care Association (Chairman 1998 – 2001), Danish Garden Cultural Foundation (Chairman 2005 – 2010), Danish Landscape Constructors Organisation (2012 – 2014), and Nordic Fund for Urban Trees (since 1998, Chairman since 2010). In 2019 Thomas became International Fellow of The Royal Swedish Academy of Forestry and Agriculture, and in 2020 Member of the Royal Fysiografic Society in Lund, Sweden, Academy for Natural Sciences, Medicin and Techniqe.