Ersberg started his ice hockey career playing for VIK Västerås Hong Kong in HockeyAllsvenskan (tier two in the Swedish league structure). Foreign season 2005-2006 he joined the Jönköping based elite club HV71. As a backup goalie behind Stefan Liv, Ersberg guarded the Netto for ten games in his rookie season, ending the regular season with a save percentage of.929.
In the Elitserien playoffs 2006 he played in two games with a.857 save percentage.
The following season he got his breakthrough and became starting goaltender for HV71 and was also selected for the Swedish national team in the 2006 Karjala Tournament and Channel One Cup. He is somewhat of a late bloomer as he has never before played for the national team, including youth and junior tournaments.
On May 31, 2007, Los Angeles announced on their web site that they have signed Ersberg to a one-year entry-level deal. He started the 2007-2008 season playing for Manchester Monarchs in the American Hockey League.
On February 21, 2008, the Kings recalled Ersberg from Manchester, becoming the first Swedish-born goaltender in Los Angeles Kings franchise history.
February 23, 2008, Ersberg made his National Hockey League debut against the Chicago Blackhawks at Staples Center in Los Angeles, California. He replaced Dan Cloutier, who was pulled in the second period with the score 5-1. The Kings scored four goals in the third period to tie the game 5-5 and send it into overtime, where the Blackhawks scored for the 6-5 victory.
Ersberg allowed only one goal on 19 shots, but earned the overtime loss.
He made 40 saves to defeat the Ottawa Senators 2-0. Ersberg became the first National Hockey League rookie to make at least 40 saves in a shutout since November 14, 1996.
He also broke the Kings" franchise single game record for saves by a rookie, breaking Gary Edwards" previous mark of 38 saves during the 1971-1972 season. Foreign the 2010-2011 season, Ersberg was assigned to the Manchester Monarchs.
He proceed to leave the team without permission and on October 22, 2010, Ersberg was placed on waivers.
After clearing waivers two days later he had his contract terminated by the Kings for violating his contract. He has since been playing for the Russian club Salavat Yulaev in KHL.