Through his photographs and actions toward saving threatened buildings, he played an important role in the preservation of Montreal's historic structures, including the Shaughnessy House, which later became the Canadian Centre for Architecture, and the Windsor Station.

His father, John Campbell Merrett, was an architect and town planner whose work included the Art Deco–styled interior of the Central Station in Montreal.

Brian Merrett became actively involved in various organizations aimed at preserving the city's architectural heritage from large-scale urban infrastructure projects.

[3][4] He played a crucial role in saving the Shaughnessy House, a historic building that once belonged to Canada's railway baron family, was at risk of demolition.

[8] He continued to remain active into the 2020 working on an effort to drive awareness to save the 169-year-old Fulford Residence in Montreal from demolition in 2021.

[13][1][6] In his activism focused on urbanism and preservation of heritage buildings, Merrett was among a group of photographer activists that included Clara Gutsche, Melvin Charney, and David Miller.

[3][14] Through his career, Merrett published several books, collaborating with Dane Lanken and, extensively, François Rémillard, that focused on Montreal's heritage architecture.

[3] Merrett donated a portion of his photographic archive to the McCord Museum to preserve his decade-long documentation of Montreal's past.

[9] Merrett was memorialised in publications by the institutions he was associated with, as well as local and national[18] media, and in a statement in the House of Commons of Canada the 8th of November, 2023, by Member of Parliament Anna Gainey.