Berndtson passed studentexamen in 1966. He graduated from the Military Academy Karlberg in 1970 and was promoted the same year to lieutenant in Bohuslän Regiment, where he was promoted to captain in 1973 and served as a platoon leader. Berndtson was educated at the Swedish Infantry Combat School in 1972 and he attended the Higher Staff Course at the Swedish Armed Forces Staff College from 1978 to 1980. He was promoted to major in 1981 and entered the General Staff Corps in 1983, and served as a teacher at the vid Swedish Armed Forces Staff College from 1984 to 1986. Berndtson was promoted to lieutenant colonel in the General Staff Corps in 1986, and studied at the Bundeswehr Command and Staff College in Germany from 1986 to 1987, and served in the Dalarna Regiment from 1987 to 1989 and was head of the Planning Department in the Defence Staff from 1989 to 1991. In 1991, Berndtson was promoted to colonel and appointed commanding officer of the Gästrike Brigade. He was promoted to colonel 1st class in 1993 and then served as commanding officer of the Gävleborg Defence District as well as of the Hälsinge Regiment from 1993 to 1994. Berndtson then served as commanding officer of the Middle Army Division from 1994 to 1996. During the 1990s he also educated himself at the Swedish National Defence College: Higher Operational Management Course in 1993, Main Course in 1994 and the Senior Management Course in 1996. In 1996, Berndtson was promoted to major general and from 1996 to 1998 he served as head of the Supreme Commander's coordination group for implementation of the Defence Act of 1996. He was promoted to lieutenant general in 1998 and was head of the General Training and Management Directorate in the Swedish Armed Forces Headquarters from 1998 to 2000. From 2001 to 2005, Berndtson served as Deputy Supreme Commander and head of the Swedish Armed Forces Headquarters, after which he retired from the Swedish Armed Forces in 2005. Berndtson has also been an expert in the Swedish Parliamentary Defence Commission, and chairman of the Swedish Armed Forces Personnel Responsibility Committee. He was also a member of the board of the Society and Defence, the Swedish Coast Guard, the Swedish National Board of Psychological Defence and the Stiftelsen Gällöfsta kurscentrum.
