Mares was born in New Orleans. His father, Joseph E. Mares, played cornet with the military band at the New Orleans lakefront and ran a fur and hide business.
Like many New Orleans cornetists of his generation, Joe Mares's main influence was "King" Joe Oliver.
About 1919 cornetist Abbie Brunies was offered a job playing in Chicago, and passed the offer on to Mares. (Brunies thought his New Orleans position of doubling driving a taxi-cab and playing music was more secure than prospects in Chicago.)
Mares established himself as a respected band leader over a group of wild and strong willed musicians, as The New Orleans Rhythm Kings became one of the best regarded bands in Chicago in the early 1920s.
In late 1924 Mares returned to New Orleans. He decided to play music on the side while taking over the running of his family fur & hide business. He ran the business well and with his prosperity purchased 3 homes for himself and his relatives in New Orleans' new suburb of Metairie, Louisiana. Mares's Metarie home was the site of a legendary jam-session in 1929 where Bix Beiderbecke and the other jazz playing members of the Paul Whiteman Orchestra jammed with the local New Orleans jazz musicians.
Mares also ran a restaurant in New Orleans called "The Chicago Bar-B-Q". In the early 1930s he returned to Chicago where he opened up his "New Orleans Bar-B-Q" there. The "P.M. New Orleans Bar-B-Q" became a gathering place for Chicago jazz musicians and home to numerous jam sessions, which Mares occasionally joined in.
Mares's 1936 recording session for Okeh Records show his style had not remained static; he sounds more under the influence of Henry "Red" Allen than Joe Oliver.
Paul Mares died at the age of 49 of lung cancer, according to his brother Joe, caused by "smoking too many cigarettes".