Diagna won the 400 metres hurdles gold medal at the 1997 World Championships in Athens and the 4 x 400 metres relay gold medal at the 2003 World Championships in Paris. In 2002 he won the 400 metres hurdles gold medal at the European Championships in Munich. Diagana also set a new, European 400 metres hurdles outdoor record of 47.37 sec. in Lausanne, Switzerland on 5 July 1995. This record stood until June 2019.
In his only Olympic appearance, Diagana finished in fourth position in the final of the 400 metres hurdles of the 1992 Olympics in Barcelona. In that Olympics, his 4 x 400 metres relay team was eliminated in the heats.
On 21 January 2011, Diagana was seriously injured in road accident while he was cycling along a road of the Col de Vence in the department of Alpes-Maritimes in southeast France. He was airlifted by helicopter to a hospital in Nice. He suffered head injuries and although in a serious condition, he did not lose consciousness.
He is married to Odile Lesage, with whom he has three children.