It serves Saint-Hyacinthe its surrounding Les Maskoutains Regional County Municipality, as well as the neighbouring Acton Regional County Municipality and La Vallée-du-Richelieu Regional County Municipality.

It currently has 213 acute care, 35 short-term psychiatric and 25 mental health shelter beds (Saint-Charles Pavilion).

[1] The hospital is named for Honoré Mercier, a lawyer, journalist and politician best known for being the ninth Premier of Quebec from 27 January 1887 to 21 December 1891.

[2] From June 2001 to April 2007, the hospital was renovated to correct water infiltration problems in the building.

[4] In 2004, the Hervé-Gagnon Pavilion opened, named for a longtime doctor of the hospital.