He commanded the 10th Infantry Brigade during the Fenian Raids and was awarded a medal for his conduct.
He served as aide de camp to Lord Stanley of Preston and to Lord Aberdeen, was colonel of the 61st Regiment in 1903 and was the regiment's Honorary Colonel in 1909.He won a seat in the Quebec Legislative Assembly in 1875 but the election was overturned a year later.
He served as president of the Conservative Party Association of Quebec for several years and was first elected to the House of Commons of Canada in 1878 as a Conservative representing Montmagny, Quebec.
He was re-elected in the 1882 election but was defeated in 1887.In 1892 he was summoned to the Senate of Canada on the advice of Prime Minister John Joseph Caldwell Abbott and sat as a Conservative.
He also served as Mayor of Limoilou in 1899.
After the 1911 federal election brought the Conservatives into power after a fifteen-year absence, the new prime minister, Robert Laird Borden, appointed Landry to be Speaker of the Senate.Landry was a prolific author and editor who wrote a number of books on matters of public policy in addition to his writings on agriculture.
He was particularly concerned with the rights of French Canadians living outside of Quebec, particularly Franco-Manitobans and Franco-Ontarians and their rights in regards to the Manitoba Schools Question and a similar debate on publicly funded francophone schools in Ontario.His interests in these questions made him a controversial figure as Speaker when the Senate was considering the issue of francophone schools in Ontario in the wake of the provincial Conservative government's passage of Regulation 17 which restricted French-language instruction.
Many of Landry's rulings were challenged on the floor of the Senate with several being put to a vote in a departure from the normal practice of accepting a Speaker's ruling as final.
In the face of his diminished authority over the upper house and the fact that many of his rulings were being overturned, he absented himself from the Chair for several days which resulted in calls for the government to dismiss him if he did not resume the Chair.
Upon his return, explained that he had absented himself due to the opposition by a number of government members to his rulings and the lack of support he enjoyed.
Landry continued as Speaker for a further year before resigning.
He remained a Senator until his death three years later in Quebec City and devoted much of his time writing and campaigning on the issue of French-language rights in Ontario.