As a youth player Stranzl played for SV Güssing. In 1997 he earned a transfer to TSV 1860 München, where he completed his youth tranining and debuted as a Bundesliga professional. In 2004, following TSV's relegation, the defender joined fellow first-divisioner VfB Stuttgart, where he remained a team cornerstone.
In March 2006, Stranzl signed for Russian Premier League side Spartak Moscow. In February 2010 Spartak Moscow received an offer from AC Omonoia, a football club from Cyprus, to transfer him at the end of the season in Cyprus. Spartak rejected the proposal but negotiations continue between the two teams.
On 30 December 2010, Stranzl agreed to terms with Borussia Mönchengladbach on a two and a half-year deal. He helped the side avoid relegation. At the time of his signing, Borussia Mönchenglabach was last in the Bundesliga.