Diouf, who was born in Chad but had French and Senegalese citizenship, led the club between 2005-2009 and helped build the side who lifted the Ligue 1 title in 2010.
He was hospitalised in Senegal after contracting coronavirus there and became the country's first COVID-19 fatality.
Diouf had been due to leave for Nice earlier on Tuesday to be treated in France, but a sharp deterioration in his health -- which saw him placed on a respirator -- prevented him from boarding the plane.
Diouf was also a journalist and football agent before taking over at the Stade Velodrome.
Moving to Marseille aged 18, he was set on a career in the military, but soon switched paths.
After studying at the prestigious Sciences Po in Paris, he worked at the La Marseillaise newspaper before becoming a football agent, most notably for Didier Drogba, who enthralled the Velodrome in 2003-04.