Muller was one of the leading lock forwards in South Africa when he joined Ulster Rugby ahead of the 2010-11 season from the Natal Sharks.
Standing at 6'7"and 114kg, he made his provincial debut for the Sharks in 2003, going on to make his Super 14 debut against the Hurricanes later that season. Johann made over 130 appearances for the Sharks and captained the side during the 2008 and 2009 Super 14 competitions. He led the Sharks to Currie Cup victory in 2008.
Muller was awarded his first Springbok cap against Scotland in 2006 and captained South Africa against New Zealand in 2007. He has represented his country on 24 occasions and was a member of the squad which won the 2007 Rugby World Cup in France.
Since coming to Ulster, Johann has had a remarkable influence both on and off the pitch. His leadership ability was instrumental in helping Ulster reach the Magners League semi finals and Heineken Cup quarter finals in 2010/11.
His performances did not go un-noticed as he picked up the URSC player of the Season Award, which also highlighted his popularity among supporters. At the Ulster Rugby Awards Dinner in May 2011 he picked up the biggest award - Personality of the Year - which was again a tribute to his work on and off the field.
Shortly after the awards dinner he signed a contract extension keeping him at the club until the summer of 2013.