His name is listed with the World War 1 Veterans on the Front of the Valleyfield Cenotaph, but I cannot find any service record that ties a specific individual to the area. There are no Peter Mannings in the Libraries and Archives Canada files for the Canadian Expeditionary Force, and there is 1 listing for a young man in the Royal Newfoundland Regiment and Forestry Corps, but I cannot find a tie to this man in Valleyfield. He was young, english speaking and had never been on the Montreal Cottons payroll so it seems unlikely that this refers to the Newfoundland Peter Manning.
There is one Private Peter Manning who is on the Canadian Virtual War Memorial site. He served with the East Lancashire Regiment, 1st Battalion, Regimental number 7913. There is no information on why he is on the CVWM site with respect to his relationship to Valleyfield, but there are many ties between Lancashire and Valleyfield, so he is the most likely candidate.