Lambert presided over the House of Commons during a tenuous minority government situation. As speaker, Lambert strove to be very correct in his interpretation of standing orders, ruling opposition questions out of order during Question Period if they were not strict inquiries and strayed at all into argumentation. This displeased the Opposition and led to his rulings being appealed unsuccessfully.
Lambert refused to allow an emergency debate on the issue of Bomarc missiles that the opposition demanded when an American State Department press release was issued contradicting arguments made by the Diefenbaker government against accepting the missiles. Lambert asserted that the matter was not of sufficient urgency to warrant a special debate. Liberal leader Lester Pearson challenged Lambert's decision, and the House overturned Lambert's decision by a vote of 122 to 104.
This incident indicated that the government had lost control of the House. Soon after, the government was defeated on a motion of no confidence on the Bomarc issue. Diefenbaker called an election, and appointed Lambert to Cabinet as minister of Veterans Affairs. While Lambert was re-elected in Edmonton, the Progressive Conservative government lost the election, and Lambert's two-month career as a cabinet minister came to an end.
With the Conservatives in Opposition, Lambert sided with Diefenbaker's critics, and refused to sign a petition declaring loyalty to the Conservative leader in 1966 when Dalton Camp attempted to force a leadership review.
In Opposition, Lambert was a leading critic in the areas of National Defence and Finance. During the short-lived government of Joe Clark, he was chairman of the Miscellaneous Estimates Committee, and led it through a flurry of activity.
After being defeated in a bitter nomination race by his eventual successor, Murray Dorin, Lambert retired from the House of Commons at the 1984 election. He was appointed to the Canadian Transportation Commission by Prime Minister Brian Mulroney following the election.