During his twenty-five year tenure at Hallwalls Contemporary Arts Center in Buffalo (2001—2026), Massier oversaw exhibitions, and projects by hundreds of artists, including Suzy Lake, Laylah Ali, Julio Cesar Morales, Ruben Ochoa, Betty Tompkins, Jacob Kassay, Guy Richards Smit, Wayne Hodge, Jillian McDonald, Sam Van Aken, Kelly Richardson, Christina West, Kara Tanaka, Joan Linder, George Afedzi Hughes, Ester Partegas, Shary Boyle, Deborah Aschheim, David Kramer, Katie Bell, Alexandria Smith, Kenny Rivero, Shea Hembrey, and Emily Vey Duke & Cooper Battersby, among many others.

Prior to joining the staff of Hallwalls in early 2001, Massier had been a curator and writer in Toronto since 1988. Through the 1990s, at The Koffler Gallery, he curated more than sixty exhibitions of emerging and mid-career Canadian artists.

He has written feature articles, catalogue essays, artists’ profiles, as well as exhibition and book reviews for various galleries and visual art publications, including Canadian Art, MIX magazine, Coagula Art Journal, THIS magazine, Art in America, Art Papers, the Buffalo News, UB Art Galleries, the Burchfield Penney Art Center, and the Albright Knox Art Gallery. In 1997, he co-founded the Toronto art publication LOLA.

In 2010, he served as Project Director for the regional survey biennial Beyond/In Western New York 2010: Alternating Currents.

In March 2026, he joined the Burchfield Penney Art Center as their Chief Curator.