His family moved to Madrid while he was still very young and he obtained his degree in history and political science at Complutense University, where he studied with José María Jover. While there, he was a member of the "Union of Democratic Students" and the "Union of Young Christian Democrats"; groups which opposed the Francoist "". He has been a teacher since receiving his doctorate in 1966; beginning as an assistant professor at his alma mater. In 1975, over some opposition, he was named an Aggregate Professor of modern history at the Autonomous University of Barcelona. Two years later, he obtained a professorship at the University of Valencia. In 1981, he became a professor at UNED; a position he held, with a brief interregnum, for the rest of his life. Attracted to politics, he joined the new Democratic People's Federation, led by José María Gil-Robles, in 1975. After the party's dissolution in 1977, he switched to the Christian Democratic Party. In the first Post-Franco municipal elections, he was elected a councilor in Madrid on the UCD ticket. From then until 1982, he was also Director General of the "Patrimonio Artístico, Archivos y Museos" at the Ministry of Culture. At this time, he oversaw negotiations between the family of Pablo Picasso and the Museum of Modern Art to return Picasso's iconic painting, Guernica, to Spain. Its installation at the Museo del Prado would become a major symbol of the restoration of democracy. Despite this success, Tusell was dismissed by the Minister of Culture, Soledad Becerril, ostensibly over disagreements involving libraries and restoration procedures, but possibly because she felt that he had overstepped his authority. The dismissal, delivered by telephone, was protested by Joan Miró, Pablo Serrano, Antoni Tàpies and Eduardo Chillida. After briefly being a member of the People's Democratic Party, he left politics and returned to his professorship duties at UNED. In 1999, the Council of Ministers appointed him as their representative to the "Fundación Colección Thyssen-Bornemisza". Apart from his activities as a teacher and writer, he was also a major contributor to periodicals such as El Mundo, El País, La Vanguardia and the now defunct Diario 16, as well as to the radio networkSER. At the beginning of 2002, he was diagnosed with leukemia, which led to his death three years later. He was married to the historian, Genoveva García Queipo de Llano and had two children.
