Feldt was the son of a single mother, Irma, née Jonsson, who had to send young Kjell-Olof to live with his grandfather's sister because of his father's alcoholic problems.

Though he came from a working-class family, Kjell-Olof managed to gain admission to Uppsala University where he received a Politices Magister (extended BA in political science) degree in 1956.

"[1] In the late 1980s, Feldt was heavily criticised from within his own party: he and others at the Ministry of Finance Sw. "kanslihushögern" were perceived to be promoting right-wing politics and to be failing to live up to the traditional ideals of the social democrats.

When economic problems mounted in 1990, the rift was highlighted, and Feldt left office after a fall-out with Prime Minister Ingvar Carlsson on 16 February.

Feldt had been in favour of a more conservative economic policy in response to the crisis, and when his ideas met resistance, he decided to leave his office.