He also became International Policy Secretary of the Federal Committee of the PSOE and vice-chair of the Union of Socialist Parties of the European Community, as well as being a member of the Parliamentary Assembly of the Council of Europe from 1 January 1978 to 1 April 1983.

Following the Socialist success in the Spanish general elections of 1982 Marín joined the government as State Secretary for Relations with the European Communities – effectively, as Chief Negotiator for Spain's admission to the European Communities, a politically visible appointment because successful negotiations were seen as a seal of approval of Spain's transition from a dictatorship under Franco to a genuine parliamentary democracy.

Marín was given the portfolio of Social Affairs, Education and Employment – subjects which until his arrival (the mandate of the first Delors Commission had begun the previous year in 1985) had been handled by Peter Sutherland.

Marín was reappointed into the second Delors Commission from 1989 to 1992, again as vice-president but this time with a portfolio which interested him more: development cooperation and the Common Fisheries Policy.

Marín was one in a field of four candidates for two vice-president posts, and was elected behind Leon Brittan, defeating Édith Cresson and Martin Bangemann.

His initial portfolio in this mandate was External relations with Southern Mediterranean countries, the Middle East, Latin America and Asia (except Japan, China, South Korea, Hong Kong, Macao and Taiwan) including development aid; in this period difficulties in implementing the EU's "Mediterranean strategy" under his leadership began to lead to complaints -never confirmed- of incompetence and of fraud.

Following the biting report of the group,[5] which (among other things) criticised Marín for his lack of responsibility, the Santer Commission resigned en bloc on 15 March 1999.