Salas was born in Phnom Penh on 21 October 1937. He studied in Paris for his priestly formation and was ordained a priest in 1964. His first assignment was in the apostolic prefecture of Battambang, but he soon returned to France for further studies.

In April 1975, the Khmer Rouge seized power in Cambodia. They founded a communist state under the name Democratic Kampuchea: all religious expression was forbidden and the destruction of places of worship was frequent. In May 1975, every foreigner in Cambodia was expelled including Catholic priests and religious, while the natives were forced to work in the rice fields and many of them were executed.

French bishop Yves Ramousse was at the head of the Cambodian Church when the Khmer Rouge seized power. Since the prediction was that he would be expelled from the country for being a foreigner, he called Salas back to Cambodia. On 14 April 1975, the Holy See appointed Salas as Coadjutor Bishop for the Apostolic Vicariate of Phnom Penh. On 30th April, Bishop Ramousse was expelled from the country with many other foreign priests and religious. Most of the Cambodian priests and religious remained in the country, very few would survive.

In 1976, Bishop Ramousse resigned as head of the Cambodian Church. Salas took over its leadership, but was sent by the Khmer Rouge regime to a rice camp in Kompong Thom. He died of exhaustion in September 1977 in the Traing Kork pagoda. On 1 May 2015, the Cambodian Catholic Church officially opened an investigation into the alleged martyrdom of Joseph Chhmar Salas and 33 others who died during the Khmer Rouge regime.