Thomas Andersen joined the Norwegian Navy in 1993 after completing the Non-commissioned officer's training in the Coastal Artillery. He began his career as rifle platoon commander before joining the Submarine Service in 1997. Andersen began his submarine career on the Kobben-class submarines and later moved on to the Ula-class. The time in the Submarine Service was interrupted by two years serving in the Coastal Ranger Fast Insertion Craft (Combat Boat 90N) squadron. Andersen left the Submarine Service in 2008 after serving as commanding officer from 2005.

Andersen has since served as staff officer in the Norwegian Naval Special Operations Command (2008-2009), staff officer Submarines at Norwegian Joint Headquarters (2009-2012) and staff officer in the Norwegian Intelligence Service (2013-2015). In 2015 he served in the Department of Security Policy and Operations in the Norwegian Ministry of Defense. In 2017 Andersen moved to Washington D.C for a posting in US Joint Staff J5 Global Integration Division as Norwegian liaison officer. Since returning to Norway in 2021 he has served at N5 and DCOS OPS in the Norwegian Navy. In December 2023 Andersen was assigned as co-lead for the UK/UKR/NOR-lead Maritime Capability Coalition for Ukraine.
Cdre (act) Andersen graduated from the Norwegian Naval Academy in 1997, the US Naval War College in Rhode Island in 2013 and attended the Senior Executive Course at the Norwegian Defence University college in 2023.

Andersen did several deployments in Operation Active Endeavour while serving in the submarine service and had one tour in Afghanistan as Norwegian liaison to ISAF SOF.

In his spare time Thomas enjoys fly fishing, kayaking and still hopes to use the scuba gear stowed away in the back of the closet. As a consequence of enjoying to cook, he may also be found hiking and jogging.