Red Bull Salzburg Janko began his career at
Admira Wacker in
Mödling,
Lower Austria. After successful years, he joined
Red Bull Salzburg. He was one of the most important players for the team, and in the end of the 2004–05 season, he scored 11 goals in 10 matches.
2008–09 season Janko began the
2008–09 season scoring five goals in the first two matches. On 16 November 2008, he became the all-time club leader for goals scored in one season for Salzburg. His 25 goals in less than half a season, surpassed previous record-holder
Oliver Bierhoff, who scored 23 during the
1990–91 season. Janko continued his scoring streak with a four-goal performance against league rival
Altach. On 6 December 2008, he surpassed
Toni Polster as the all-time half-season scoring champion by scoring 30 goals in 20 matches. Janko scored five hat tricks in the league during the season, including a four-goal performance after coming on as a second-half substitute against
SCR Altach in a 4–3 victory. In total, he scored 39 goals (plus 9 assists) in 34 matches as Red Bull Salzburg won the league. Due to his 39 goals scored in the 2008–09 season, he became the topscorer in all European leagues during the year. His success caught the attention of several English
Premier League clubs and
Celtic. However, he signed a new contract with Red Bull on 30 January 2009, which would have kept him in Salzburg until June 2013.
2009–10 season Though there were rumors and speculation that Janko would sign with a different club, he stayed with the Red Bulls. In 21 matches that season, Janko scored 12 goals which currently put him in second place in the Bundesliga behind teammate
Roman Wallner. His best performances were a four-goal effort against
FC Kärnten on 4 October, and a pair of goals scored against
Josko Ried on 13 February 2010. Janko was also an integral part of the Salzburg team that enjoyed success in its pool play in the
UEFA Europa League. He had a strong performance against
Standard Liège on 19 February 2010, scoring his side's two goals in a 3–2 loss in the first tie of the round of 32.
Twente Janko signed a four-year contract with Dutch club
Twente on 21 June 2010 and was given the number 21 shirt, previously worn by fellow Austrian
Marko Arnautović. Twente paid Red Bull €7 million for Janko's signature.
2010–11 season Janko scored his first goal for his new club on 21 August 2010, his side's first in a 3–0 win over
Vitesse in the third match of the
Eredivisie season. In the club's next Eredivise match, Janko scored a brace and notched an assist as Twente defeated
Utrecht 4–0. Janko scored four goals for Twente in a 5–0 rout of
Heracles on 19 January 2011, while also providing an assist for
Luuk de Jong's goal. On 23 January, he scored both goals for Twente as they came from a goal down to defeat
Groningen. On 8 May 2011, Janko scored the winning goal of the
2011 KNVB Cup final against
Ajax in the 117th minute to seal a 3–2 victory.
2011–12 season In the
2011 Johan Cruyff Shield on 30 July 2011, Janko scored from the penalty-spot as Twente defeated reigning Eredivisie champions Ajax 2–1 to lift the cup. In the first match of the
2011–12 Eredivisie season, Janko scored the match's only goal as Twente defeated
NAC Breda. Janko scored twice on 28 August, scoring Twente's third and fifth goals as they defeated
Heerenveen 5–1. On 29 September, he scored twice to lead Twente to a first victory in the
UEFA Europa League, a comfortable 4–1 win over
Wisła Kraków. On 15 October, Janko scored a hat-trick as Twente defeated
RKC Waalwijk 4–0 in an Eredivisie match. On 1 December, he scored a goal in the final minute of normal time as Twente defeated
Fulham 1–0 to secure first place in Group K of the
UEFA Europa League group stage.
Porto On 30 January 2012, it was announced that Janko would sign for
Porto. On 31 January, Janko was officially unveiled as a Porto player and was given the number 29 shirt, joining in a €3 million move and signing a contract until June 2015. On 5 February, he scored his first goal for Porto against
Vitória de Setúbal in the
Taça de Portugal, a 2–0 win.
Trabzonspor On 28 August 2012, after just half a year at Porto, Janko signed for Turkish club
Trabzonspor for a €2.4 million transfer fee. He made his debut for the club five days later, coming on for
Paulo Henrique in the second half of a 0–1 loss to
Gaziantepspor. Janko finally scored his first goal for Trabzonspor on 19 November, opening the scoring for the club in their 2–1 victory over
Orduspor. Janko scored his first goal of the
2013–14 season on 24 October 2013, scoring Trabzonspor's goal in a 2–0 victory over
Legia Warsaw in the
UEFA Europa League group stage. On 4 December, he scored against
Balıkesirspor in the fourth round of the
Turkish Cup, but the goal was not enough as Trabzonspor fell to a 1–3 defeat. He scored just his second league goal during his time in Turkey on 23 February 2014, scoring the match winner against
Kayserispor. At the end of the season, Janko was released from his contract, allowing the forward to move to a new club on a free transfer.
Sydney FC On 31 July 2014, Janko was announced as
Sydney FC's new marquee signing for the
2014–15 A-League season, on a one-year deal, meaning his wages were permitted to be paid outside the league's salary cap. On 30 August, he scored his first goal for Sydney during the final of the inaugural Townsville Football Cup in a 2–0 win against
Brisbane Roar. On 24 October, he scored his first A-League goal, against Brisbane Roar in a 2–0 victory. His 35-yard shot bounced into the top left corner to give Sydney a 1–0 lead. Janko scored his next two goals in the form of a
brace against
Melbourne City, after having turned down the opportunity to play for the Austria national team against
Brazil. He scored another two goals in Sydney's 5–1 rout of
Central Coast Mariners on 24 January 2015. Janko was awarded the
Professional Footballers Australia Player of the Month for February following a run of six goals in four matches during the month. Janko scored his first
hat-trick for Sydney in their 5–4 win against Brisbane Roar on matchday 21 at
Allianz Stadium in rather inclement conditions. His hat-trick also saw him become Sydney's highest season goalscorer with 16 goals (previously held by
Alessandro Del Piero during the
2012–13 A-League season with 14). In addition, Janko became the first ever player to score in seven consecutive matches, the previous record being held by ex-Brisbane Roar player
Besart Berisha with six. On 26 May 2015 it was announced that Janko would be leaving Sydney due to difficulties with attending league matches and international duty.
Basel On 25 June 2015, it was announced that Janko joined Swiss side
Basel on a free transfer, signing a one-year deal, with an option for a furtheryear. He joined Basel's first team for their
2015–16 season under head coach
Urs Fischer. After playing in one test game, Janko played his
domestic league debut for the club in the away game in the
Letzigrund, in
Zürich, on 25 July 2015. He also scored his first goal for the team in the same game as Basel won 3–2 against
Grasshopper. Janko scored a hat-trick in the home game in the
St. Jakob-Park on 13 March 2016 as Basel won 4–2 against
St. Gallen. Under trainer Fischer, Janko won the
Swiss Super League championship at the end of the
2015–16 Super League season. For the club it was the seventh title in a row and their 19th championship title in total. In summary, Janko scored 16 goals and gave two assists in 20 league games in his first season for the Swiss club. He was third in the scorers list behind
Guillaume Hoarau (18 goals) and
Mu'nas Dabbur (19 goals). Janko led the scorer list for a long time. However, he tore a muscle fiber in mid-April 2016 and missed the rest of the season due to injury. In addition, the striker reached the last 16 of the
2015–16 Europa League with Basel, scoring two goals and giving two assists in that competition. The contract option was met and Janko stayed another year with the club. At the start of the
2016–17 Super League season, FCB strengthened their attacking formation, signing Ivorian player
Seydou Doumbia from
AS Roma. The two strikers were used alternately throughout the season. At the end of
Basel's 2016–17 season, Janko won the championship with the club for the second time. For the club this was the eighth title in a row and their 20th championship title in total. They also won the
2016–17 Swiss Cup, defeating
Sion 3–0 in the final. The contract between Basel and Janko was not extended by the club and he left FCB at the end of the season. During his two seasons with the club, Janko played a total of 76 games for Basel scoring a total of 37 goals. 44 of these games were in the
Swiss Super League, five in the
Swiss Cup, 17 in the UEFA competitions (Champions League and Europa League) and ten were friendly games. He scored 29 goals in the domestic league, five in the cup, three in the UEFA competitions and the other three were scored during the test games.
Sparta Prague On 6 June 2017, Janko signed a two-year contract with Czech club
Sparta Prague.
Lugano On 6 February 2018, Janko signed a contract with Swiss club
FC Lugano. ==International career==