Haas began his career at Grasshoppers Zurich, where he played for several seasons before moving to England to join Sunderland. Despite making 27 appearances for the club in 2001-02, he did not play for Sunderland during the following season and was loaned out to FC Basel.

Haas joined West Bromwich Albion in 2003,[1] and was a regular at right-back as Albion were promoted back to the Premiership. During this time, Bernt was affectionately known as "Vindaloo" due to a combination of his first and last names. He scored in the 2-0 League Cup win against Manchester United,[2] having already scored in an earlier round against Brentford.[3] He also scored once in the league against Crewe.[4] However he found his chances limited in the top-flight, and left the club by mutual consent on January 21, 2005.[5] The following day he signed for SC Bastia in France, whom he played for prior to joining Cologne. After just one season at Cologne, he joined Swiss Super League club FC St. Gallen in 2007.

Haas played for the Swiss national team at Euro 2004 and was sent off in the match against England.[6]