In 1726, he became organist at the Saint-Jacques-la-Boucherie church in Paris. Later, he is appointed organist at Saint-Nicolas-du-Chardonnet church.
He married Geneviève Françoise Legris on February 18, 1734, and they raised seven children. He was the father of musician Hélène-Louise Demars.
As composer he wrote several spiritual songs for the female students of the Saint-Cyr school, near Paris. Fétis wrote that he published an organ book now lost.
We know little else than he was the elder brother of Charles Demars (28 May 1702 - 4 Mars 1774), nicknamed « le cadet », who became in 1728 organist in the Vannes Cathedral, in Brittany, until his death.
In 1735, Charles Demars publishes his 1er Livre de Clavecin. This book contains 4 suites in the Handel manner.
This collection consists of four suites:
- Suite I in A major
- Suite II in G minor
- Suite III in D minor
- Suite IV in C minor
- IMSLP Charles Demars : Pièces de Clavecin
Two pieces by Charles Demars played by harpsichordist Claude Nadeau :