Franz-Josef Thelen finished his apprenticeship as qualified tax clerk in 1972 and studied business administration at Fachhochschule Niederrhein. He obtained his degree in 1976. After being appointed tax agent in 1981, he was appointed tax adviser in 1986.

After several years of working for one of the biggest accounting firms in Mönchengladbach he founded his own office in 1981. Since then he has been advising entrepreneurs, free lancers and private clients on all areas of taxation

In addition, he has been member of the board handling the final exams of the apprentices for qualified tax clerks.

Franz-Josef Thelen is married and has three children.