[2] It was not until 2003 when then-Mönchengladbach manager Ewald Lienen named him as an unused substitute in a Bundesliga fixture of the club in a 2–0 loss against Hannover 96 on 19 September 2003.

[3] Ironically, this turned out to be Lienen's final game in charge of Mönchengladbach and Jansen returned to the U19 side.

The start of the 2004–05 season saw Jansen playing for Borussia Mönchengladbach II in the fourth division under manager Horst Köppel.

[11] In the 2005–06 season, Jansen continued to be a first-team regular, playing in both left-back and left-midfield and his impressive performance earned him a call-up from the German senior team.

Following the 2006 FIFA World Cup, Jansen was linked with a move away from Borussia Mönchengladbach, with top-flight clubs in Germany keen on signing him.

In March 2007, Bayern Munich expressed interest in Jansen following his performance for both Borussia Mönchengladbach and the Germany national team.

[28] Jansen made his Bayern Munich debut in a friendly match, coming on for Philipp Lahm, in the 46th minute of a 4–0 win over Swiss side FC Schaffhausen.

[29] Jansen scored his first goal in his Bayern career in a friendly match in a 13–0 win over FC 07 Albstadt on 15 July 2007.

[30] He made his Bundesliga debut for Bayern on the opening day of the 2007–08 season in a 3–0 win over Hansa Rostock.

In the 2008–09 season, Jansen found himself on the bench for Bayern's opening two matches of the new Bundesliga campaign after falling out of favour with manager Jürgen Klinsmann, due to their differences.

[38] On 28 August 2008, Hamburger SV announced Jansen had signed a five-year contract until 2013 for an €8 million transfer fee.

[66][67] After returning to the first team from injury, Jansen began playing in the midfield position, either at left-wing or left-midfield for most of the season.

[78][79] After returning to the first team, Jansen then scored his first goal of the season on 28 September 2013 in a 2–2 draw against Eintracht Frankfurt.

[84][85][86][87] Despite this, Jansen returned to the first team and played in both legs of the relegation play-off matches against Greuther Fürth, where a 1–1 draw kept Hamburg retaining their Bundesliga status via an away goal.

Having previously played for the youth side, Jansen was called up by Germany under-21 national team for the first time in November 2004.

[106] He made his debut in that match,[107] coming on as a half-time substitute for Thomas Hitzlsperger as Germany lost 2–0.

[109] After appearing in further internationals for Germany, Jansen was subsequently named in the final 23-man German squad for the 2006 FIFA World Cup.

[111] After some below-par performances prior to the tournament, manager Jürgen Klinsmann benched him and put Lahm in at left back again.

With Lahm in good form, Jansen was restricted to only one appearance throughout the tournament, Germany's bronze medal-winning match against Portugal.

[112] Jansen eventually earned a recall for several UEFA Euro 2008 qualifiers, including the 6–0 hammering of San Marino in which he scored his first international goal.

[113] Despite fears of injury he sustained against Serbia in a friendly match on 31 May 2008, Jansen was nevertheless a part of Joachim Löw's squad for the main tournament.

[116] He was then benched in Germany's 2–1 loss in the second group stage match against Croatia, but suffered a shoulder injury.

[120] In the final, Jansen came on as a second-half substitute for Lahm, as he played and defended for the remainder of the match, which saw Germany lose 1–0 against Spain.

[129] Germany's first competitive game after the World Cup in early September 2010, a 1–0 win in a Euro 2012 qualifying match against Belgium, was his next international appearance.

[131][132] As a result, Jansen had not been called by the national for almost three years, including missing out on the Germany squad for Euro 2012.

[133] On 24 March 2013, Jansen again received an invitation for the Germany national team when he was re-nominated for the 2014 World Cup qualification match against Kazakhstan.

[138] On 5 March 2014, Jansen played in a friendly match in the Mercedes-Benz Arena in Stuttgart against Chile in the starting XI; he was injured in the 28th minute and replaced by Marcel Schmelzer.