In April 1991 Korthals Altes announced that he wanted to return to the Senate. After the Senate election of 1991 Korthals Altes was elected again as a Member of the Senate, he resigned as a Member of the House of Representatives the day he was installed as a Member of the Senate, taking office on 11 June 1991 serving as a frontbencher chairing several parliamentary committees. Korthals Altes also became active in the private sector and public sector and occupied numerous seats as a corporate director and nonprofit director on several boards of directors and supervisory boards (Unilever, KPN, Randstad Holding, Arcadis, Carnegie Foundation, Stichting INGKA Foundation, and the Institute of International Relations Clingendael) and served on several state commissions and councils on behalf of the government. Following the Senate election of 1991 Korthals Altes was selected as Parliamentary leader of the People's Party for Freedom and Democracy in the Senate, taking office on 13 June 1995. Korthals Altes was nominated as President of the Senate following the appointed of Herman Tjeenk Willink as Vice-President of the Council of State, taking office on 11 March 1997. In September 2001 Korthals Altes announced his retirement from national politics. He resigned as President of the Senate and a Member of the Senate on 2 October 2001.
With his resignation from the senate in 2001, he was nominated as Minister of State. Earlier in 1997, the VVD gave him an honorary membership. The Dutch Queen nominated Korthals Altes, alongside Rein Jan Hoekstra (CDA), as informateur, after a first round of talks between the CDA and Labour Party (PvdA) to form a new cabinet failed. The second Balkenende cabinet between the VVD, CDA and D66, was installed in May 2003.
Korthals Altes chaired a commission in 2007 that looked into the Dutch election process. The final report of the commission advised the government to abandon electronic voting machines, as they lack a paper trail.
