In 1872 Mountfort became a founding member of the Canterbury Association of Architects, a body which was responsible for all subsequent development of the new city. Mountfort was now at the pinnacle of his career. Mountfort notably altered the use of a segmented arch rather than one in the Romanesque style; the latter of which was considered by Augustus Pugin to be fundamentally important to the Gothic style.
In 1872 Mountfort played a pivotal role as co-founder of the Canterbury Association of Architects, which was responsible for shaping the city's future. With his career in full swing, Mountfort introduced an innovative approach to arch design, diverging from traditional Romanesque styles that Augustus Pugin held in high esteem.