After the war end he completed his studies in economics and subsequently he started working for the Finnish Ministry of Foreign Affairs.

[2] As a high-profile department manager, Horn could act relatively independently reporting directly to Minister of Foreign Affairs Ahti Karjalainen and President Urho Kekkonen.

Horn had his hand in negotiations for significant contracts for electric locomotives and nuclear power plants with Soviet Union.

[2] Horn also took part in planning Nordek, a joint Nordic project for an own economic community, which eventually failed.

Prime Minister Mauno Koivisto led the negotiations, and during the process there came dissensions between him and Horn; this had a long-term effect on their mutual relationships.

[2] Already during the Nordek negotiations, Horn was recruited to a leading position in the multi industrial company Oy Wärtsilä Ab.

[2] Wärtsilä had opened a new facility for paper machine production in Järvenpää in 1970[5] and moving of operations was on-going when Horn started his directorship.

[2] When Horn started as manager, a previously prepared plan of building a new shipyard in Turku was launched as a project.

Wärtsilä-owned porcelain producer Arabia merged with the Swedish Rörstrand, after which Wärtsilä took control of the entire share capital.

Wärtsilä also acquired the Swedish sanitary porcelain producer IDO and diesel engine business from NOHAB.

[2] By gaining ownership of Appleton works in the United States in 1983, Wärtsilä got a foothold in the North American paper machine business.