In 1990, Jääskinen was appointed as head of the European Law Section of the Ministry of Justice in Finland. In 1995 he was appointed as Advisor and Clerk for European affairs of the Grand Committee of the Finnish Parliament. Between 2000 and 2009 and from October 2015 Jääskinen has served as Judge at the Supreme Administrative Court of Finland.
In 2009 he was appointed Advocate General of the European Court of Justice.
Jääsikinen was and is in charge in two important cases relating to water. In the case c-525/12 he gave his opinion on the term water services in the Water Framework Directive, in the case c-461/13 gave his opinion on 23.10.2014 on the term deterioration also laid down in the Water Framework Directive. Both cases are of high relevance for the protection of water.
In 2014 he gave his opinion on Google Spain v AEPD and Mario Costeja González.