After graduation from The Oslo School of Architecture and Design (AHO) in 1994, Marthinsen created his own practice in 1995. From 1996 to 1999, he worked for Div. A Arkitekter and during the following years he worked for several different architectural offices, such as Telje – Torp – Aasen Arkitekter AS, Menich Arkitekter AS, Norconsult AS and in 2016, he joined NSW as a senior architect.

During his entire career Marthinsen has achieved experience as the responsible architect in all facets of the architectural profession.

Marthinsen scope of work has varied in magnitude. It includes waiting room in the form of a little house at Aker Brygge in Oslo, different educational institutions, rebuilding of the Parliament Quarter, some major housing projects, as well as various buildings and constructions related to transport and energy.

Recently Marthinsen has worked with various architectural show castings for Porsche throughout the entire process, from regulation to implementation.