Seibert's demeanor was always serious. On the ice, this manifested itself in mature play and tremendous leadership. Off ice, it meant he was a tough negotiator in contract talks. During his second season, Seibert enlisted his father as his agent in some acrimonious negotiations with the Rangers, but any ill feelings were forgotten by the time New York won the Stanley Cup that spring, beating the Leafs 3-1 in a best-of-five final series. Eventually, though, the Rangers brass tired of Seibert's tenacious haggling and he was traded to Chicago for Art Coulter.

It was in the Windy City that Seibert established himself as one of the best defensemen of his era. He was named to the First or Second All-Star Team each year between 1935 and 1944, a feat surpassed only by Gordie Howe, Maurice Richard, Bobby Hull and Doug Harvey. Seibert was generally regarded as second only to Eddie Shore in terms of skill and rugged play, and Shore once confessed that Seibert was the only man he was afraid to fight. Defensively, Seibert was one of the best shot-blockers in the game, and he could move the puck just as quickly as anyone.

Although his career was full of great accomplishments, it was compromised by one of the worst accidents in the history of the game. On January 28, 1937, in a game against the Montreal Canadiens, Seibert and Howie Morenz chased after a puck behind the Chicago net. Seibert tied up his man on the play, but Morenz fell awkwardly into the boards, shattering his leg. Just six weeks later, Morenz died in hospital, having never recovered emotionally from the devastation of the career-ending injury. Seibert himself never really got over the trauma; whenever he was asked if he'd ever played against Morenz, he'd reply bitterly, "Yeah. I killed him."

In 1938 he led the Black Hawks to the Stanley Cup, defeating the Toronto Maple Leafs in five games. He was such a favorite with the team's owner, Major Frederic McLaughlin, that he was given part ownership of the team. However, when McLaughlin died, manager Bill Tobin refused to acknowledge this gesture and instead traded Seibert to the Detroit Red Wings. He played just 43 games over a season and a half in the Motor City before he retired.

When Eddie Shore asked Seibert to coach his Springfield Indians, the team he'd begun his career with, Earl accepted. The relationship, however, was perpetually strained, and Seibert left after only one season, bitter for the experience and wanting to have nothing more to do with hockey. In 1963 he was elected to the Hockey Hall of Fame and, along with his father, who was elected in 1961, became the first father-son combination ever elected as players.