Toivonen completed his PhD on data mining in 1996.[11] He has carried out research for Nokia in 1990–1993 and 1999–2003, and at the University of Helsinki 1993–1999 and 2002 onwards.
Toivonen has been a professor of computer science at the University of Helsinki since 2002. In 2005–2006, he visited the Albert Ludwig University of Freiburg, Germany. Toivonen was head of the Department of Computer Science in 2007–2009 and vice-dean for academic affairs of the Faculty of Science in 2018-2021.
^"Kotimaiset jäsenet" [Finnish members]. Finnish Academy of Sciences and Letters (in Finnish). Archived from the original on 1 October 2016. Retrieved 8 November 2016.
^"Jäsenmatrikkeli" [Members]. Finnish Academy of Technology (in Finnish). Archived from the original on 29 June 2019. Retrieved 8 November 2016.