Mattar began exhibiting his work in numerous group shows for young painters in Beirut between 1985 and 1990. A member of the Lebanese Association of Painters and Sculptors since 1992, he is also an established art educator, having served as a professor of painting and drawing at institutions including LAU, NDU, UL, and USEK.

He has held many solo exhibitions at prominent venues such as Janine Rubeiz Gallery, Station Des Arts, The Art Circle Gallery, Balamand University, and LAU, as well as internationally in Lamspringe, Germany.