In 1906, the Jesuits handed over responsibility for the parish to the Diocese of Sault Ste. Marie. Mgr. Stéphane Côté, who was originally from Blezard Valley, was the first secular priest appointed to the Chelmsford parish. Immediately upon his arrival, Mgr. Côté arranged for the school built in 1889 to be replaced because it was in very poor condition. When the new École Saint-Joseph was completed, it could accommodate 133 students. Mgr. Côté arranged for three Grey Sisters from Ottawa to teach the children.
Under the direction of Mgr. Stéphane Côté, numerous parish organizations were established. The Enfants de Marie and the Dames de Sainte-Anne were among the first. One of the more important projects of the Dames de Sainte-Anne was conducting fundraising to build the church. Construction of the building began in 1912 and was completed in 1913. The church was consecrated by Mgr. Scollard, Bishop of the Diocese of Sault Ste. Marie, and the choir from Collège Sacré-coeur of Sudbury sang during the celebration.
Regulation XVII which limited French-language teaching to one hour a day imposed a new financial burden on the parish. In order to avoid this regulation, École Saint-Joseph was declared a parish school, which meant that it was no longer entitled to government grants. The parish had to support the school entirely even though many parishioners lived too far away to send their children there. The children of École Saint-Joseph supported their parents' choice and refused to stay in class when an English-speaking inspector visited.
In 1934, the Action catholique de la jeunesse canadienne-française was founded and a hall was built between the church and the rectory. This hall was the Centre Champlain, used as a youth and cultural centre for many years. In 1959, the Enfants de Marie housed Chelmsford's first library, the Pie XII parish library, in the Centre Champlain.
The Saint-Étienne mission in Larchwood, which was served by a traveling priest from Cartier, joined the Saint-Joseph parish in Chelmsford in 1948. The curate from Chelmsford served that mission until it was elevated to the status of a parish in 1963.