Born in Helsinki, Finland, Widenius went to the co-educational school Broban, which was first merged into Minervaskolan and later to Lönnbeckska gymnasiet.

He is the co-author of the MySQL Reference Manual, published by O'Reilly in June 2002; and in 2003 he was awarded the Finnish Software Entrepreneur of the Year prize.

[5][6] In 2008, Widenius established venture capital firm OpenOcean with his MySQL AB colleague Patrik Backman and early advisors Tom Henriksson and Ralf Wahlsten.

[10] After leaving Sun, he formed Monty Program Ab and forked MySQL into MariaDB, named after his youngest daughter, Maria.

According to its first announcement, "the Open Database Alliance will comprise a collection of companies working together to provide the software, support and services for MariaDB, an enterprise-grade, community-developed branch of MySQL".