Gardner was born in Enosburg, Vermont and attended Enosburg High School. He began playing baseball in the Franklin County League and attended the University of Vermont where he played baseball for three years. He was the first player out of the University of Vermont to play in the American League. Gardner was signed by the Boston Red Sox as an amateur free agent in 1908, and played hisf irst professional game on June 25, 1908.
He played most of his prime in the dead-ball era, as the third baseman on several successful Red Sox teams. While he was with the Red Sox, he played in the 1912, 1915, and 1916 World Series. Gardner homered in consecutive games of the '16 Series, including a three-run inside-the-park homer in Game 4.
He played in another World Series for the Indians in 1920. In the famous 10th inning of the final game of 1912 World Series, the same inning that included Fred Snodgrass and Chief Meyers making critical fielding mistakes and giving the Red Sox two extra outs to work with, it was Gardner who drove in Steve Yerkes with the winning run of the series.