The Maple Leafs named Armstrong the team's captain in 1957–58 as he succeeded Ted Kennedy and Jim Thomson who served as co-captains the season before. He finished fourth in team scoring with 42 points, then played his third All-Star Game during the 1958–59 season. He recorded four assists in the playoffs as the Maple Leafs reached the 1959 Stanley Cup Final, but lost to the Montreal Canadiens. With 51 points in 1959–60, Armstrong finished one behind Bob Pulford for the team lead. Toronto again reached the Stanley Cup Final where they were again eliminated by Montreal.