Bak studied at Aarhus University in Denmark, receiving an MS degree in computer science in 1988 after which he became active in designing and implementing object-oriented virtual machines. He has been awarded the 2018 Senior Dahl–Nygaard Prize.
After participating in the design and implementation of the BETA Mjølner System, in 1991 he joined the Self group at Sun Microsystems Laboratories in Palo Alto, California. During his time there, he developed a programming environment for Self and added several enhancements to the virtual machine.
In 1994, he joined LongView Technologies LLC, where he designed and implemented high performance virtual machines for both Smalltalk and Java. After Sun Microsystems acquired LongView in 1997, Bak became engineering manager and technical lead in the HotSpot team at Sun's Java Software Division where he developed a high-performance Java virtual machine.
In 2002, after returning to Aarhus, Denmark, Bak founded OOVM, a company which developed software for mobile phones. In 2004, he sold it to a Swiss company, Esmertec.
Bak co-developed the Dart programming language presented at the 2011 Goto conference in Aarhus, Denmark.
In 2017, Bak left Google and soon afterward co-founded[dubious ] a startup with Kasper Lund called Toit, which is building a new programming language called Toit and a platform for Internet of Things systems.
Bak holds 18 U.S. software patents in the field of virtual machines programming. In 2010, after Oracle bought Sun and with Lars Bak working for Google, Oracle sued Google for infringing on several software patents and amongst them was the "Interpreting Functions Utilizing a Hybrid of Virtual and Native Machine Instructions" patent filed by Lars Bak et al.