Leuenberger was born in Kräilingen, a settlement near Bätterkinden in the Canton of Bern and was raised in Solothurn.

He became friends with Willi Ritschard, a member of the Federal Council, who encouraged him to seek the position of secretary of the Confederation of Trade Unions for the Canton of Solothurn in 1973.

While serving at the national level, he remained active in the labor movement and, in 1990, ran for the presidency of the Swiss Trade Union Federation against Walter Renschler, but was defeated.

He was asked to run for a seat on the Federal Council in 1995 to replace Otto Stich, but declined.

[1] He stepped down as head of the transport workers union in 2005 in order to focus on his work at the federal level.