[4] He started at Enso-Gutzeit in sales and management roles in 1930, was appointed to the Board of Directors in 1935, and eventually ascending to the Chief Executive and Chairman's position in 1945.

Following the ceding of much of Finnish Karelia to the Soviet Union at the end of the Continuation War, these facilities were lost, and the company had to invest heavily to rebuild its production capacity from scratch.

[1][4] Lehtinen embarked on such an ambitious investment programme, that within seven years the pre-war production figures were exceeded, and by the time he retired a decade later in 1962,[5] Enso-Gutzeit had grown to be the largest manufacturer of paperboard in Europe, with sizeable presence also in other segments of the forestry and paper industries.

[5] This was initially done in order to boost the company's own export capabilities, but in time the activity grew into a sizeable business in its own right, which operates still today under its Finnlines brand, since 2016 as part of the Italian Grimaldi Group.

[4][6] During his tenure, the company built its opulent and controversial headquarters, designed by Lehtinen's friend, architect Alvar Aalto, completed in 1962 and situated in central Helsinki in a prominent position across the street from the Presidential Palace.