Kadlec started his professional career in 1. FC Slovácko and made his debut in a match against Baník Ostrava. In 2005 he joined Sparta Prague.
On 17 November 2007, Kadlec was called up to the Czech national team for a game against Slovakia, and made his debut in a 3–1 victory (scoring an own goal). He scored his first goal in a win over Scotland on 30 May 2008 (also 3–1).
On 16 June 2012, against Poland in the European Championship, Kadlec headed a ball off the line in the dying seconds, preserving the Czech National Team's spot in the quarterfinals of the tournament. The strike had beaten goalkeeper Petr Čech and appeared to be headed to the inside of the far post. Had a goal been scored Poland would have ensured a draw, and the Czech team would have been eliminated from the tournament.