He was born in Baltimore, Maryland, the youngest of seven children, attended Patterson High School, and served in the United States Marine Corps during World War II.

His only major league game as a player, on September 26, 1947, came with Washington; he was hitless in three at bats against Bill McCahan of the Philadelphia Athletics at Griffith Stadium, and handled seven fielding chances flawlessly as a second baseman.

On June 9, 1967, Ermer was promoted from the Twins' Triple-A affiliate, the Denver Bears of the Pacific Coast League, to replace Minnesota manager Sam Mele.

Needing only one win in two games at Boston's Fenway Park to clinch a tie for the championship, the Twins lost both contests to the Red Sox, who became improbable league champions.

The Twins' job (his final record: 145–129, .529) was his only Major League managerial opportunity, but Ermer also served as an MLB coach for the Baltimore Orioles (1962), Milwaukee Brewers (1970–1972) and Oakland Athletics (1977).