Roudanez was born in 1823 in St. James Parish, Louisiana. His parents were a French merchant and Aimée Potens, a free woman of color. Throughout his life, Roudanez considered himself a black man. He went to France for his higher education, receiving a medical degree there. He returned to the U.S. and enrolled in medical school again, either at Dartmouth College or Cornell University. Returning to New Orleans, Roudanez opened a medical practice.
In 1862, Roudanez founded L'Union, a newspaper published in both French and English and primarily serving the free black community of Louisiana. He followed it with La Tribune de la Nouvelle Orléans. After a feud with Northern Republicans leading Reconstruction efforts in the South and disputes over which Republican candidates to support in the gubernatorial election of 1868, he was ostracized and his paper lost support. Roudanez was part of a faction that split off from the local Republican Party over the party's nomination of Northerners. Described as "Mulattoes", those who spoke French, were of mixed heritage, were relatively prosperous, and had lived freely in the area for many years had their own concerns such as voting rights, access to schools and public institutions, and other legal protections. They were joined by some local whites and were pitted against freedmen and Republicans from the North, described derisively as carpetbaggers, including Thomas W. Conway who was a fierce critic and opponent of Roudanez. Conway traveled to Washington D.C. after the election to get the Republican Party to decertify Roudalez' paper as one of two official Republican Party publications used to print notices and official publications. Several members of Roudanez' faction were kicked out of the party after the election. In the wake of the fracas, Roudanez' paper lost standing and support. It closed in 1868 before reopening when Henry C. Warmoth, the establishment Republican Yankee candidate won. Warmoth watered down and vetoed civil rights legislation and was eventually run out of office over allegations of corruption. In 2015, an historic marker was placed on Roudanez's tomb in Saint Louis Cemetery No. 1, Alley 9.
