Son of peasants, Frutos worked until the age of 25 in agriculture, emigrating later to Germany and returning to Barcelona where he got in touch for first time with trade unionism in a textile factory in Blanes. There he was an organiser of the then clandestine Workers' Commissions(CCOO). In 1963 Frutos joined the Unified Socialist Party of Catalonia (PSUC), the fraternal party of PCE in Catalonia. In the 1970s he was elected organisational secretary of the Workers' Commission of Catalonia, and represented the union in the Assembly of Catalonia. Frutos was also a member of the Confederation Executive Committee of CCOO. In 1980 Frutos was elected a member of the Parliament of Catalonia and in 1981 General Secretary of PSUC he would remain in those positions until 1982 and 1984. During that period Frutos saw different disputes between Leninist and eurocommunists, that lead to the expulsion of the Leninist majority including Chairman Pere Andarica, that would later become the Communists' Party of Catalonia.
After the resignation of Santiago Carrillo as General Secretary of PCE and the election of Gerardo Iglesias due to the poor results in the 1982 elections, Frutos transferred to Madrid to work within the Structures of the Central Committee of PCE. After the Creation of United Left (IU) in 1986 Frutos was also a member of the Praesidium of IU. In the 12th Congress of PCE in 1988 Frutos who went was elected Organisational Secretary of the Party. In the 1993 elections Frutos was elected a deputy for Madrid, In the 13th Congress of PCE in 1995 Frutos rose to the position of Secretary of the Federal Committee, a position that substituted the position of Deputy General Secretary. In the 1996 elections Frutos was second on the Madrid List of IU and succeeded in being re-elected. In 1998 the then General Secretary of PCE and General Coordiantor of IU Julio Anguita suffered a non fatal heart stroke. Following that he announced his intention to gradually abandon active politics. In December of that year the 14th Congress of PCE was convened, with frutos elected to the position of General Secretary. In 1999 following Anguita's second stroke, Frutos was asked to be IU's Candidate for Prime Minister in the 2000 elections. Prior to the elections Frutos signed a polemic agreement with PSOE then lead by Joaquin Almunia. In those elections IU fell from 21 deputies to 8. Later in the Year Frutos unsuccessfully disputed IU's General Coordination to Gaspar Llamazares in the 6th Assembly of IU. Remaining as a deputy until 2004, Frutos was re-elected general Secretary of PCE in the 16th and 17th Congresses of PCE.