Baker is best known for having been Government House Leader during the short-lived minority government of Joe Clark. He received much of the popular blame for the defeat of the government in a Motion of no confidence on December 13 1979 with the claim that the government fell because "Walter Baker couldn't count". However, observers pointed out that targeting Baker as the scapegoat was unfair as he was House Leader, not Party Whip. The defeat was the result of the Clark government's decision to alienate the six Social Credit Members of Parliament by refusing to accord them official party status as well as Clark's view that he could "govern as if" he had a majority government.
Baker was first elected to the Canadian House of Commons in the 1972 election as the Progressive Conservative Member of Parliament for Grenville—Carleton (later renamed Nepean—Carleton) and was re-elected in the 1974, 1979 and 1980 elections. He served as both Government House Leader and Minister of National Revenue during the Clark government. He served as Opposition House Leader from 1976 to 1979 and again from 1980 to 1981.
In government, Baker introduced the first ever Access to Information Bill which died on the order paper with the Tory government. However, much of Baker's bill became part of the eventual Access to Information Act that was introduced by Liberal Solicitor-General Francis Fox in 1983 and passed by parliament into law.