At the age of nine, Hajnal joined Ferencvárosi TC. He played two matches in the Hungarian National Championship I for Ferencvárosi TC. In the same year, he moved to Dunakanyar-Vác FC as part of an exchange deal. He did not play any matches in the Dunakanyar-Vác FC. Next year he headed for the German Bundesliga.
Schalke 04
made a lot of money by quickly selling him to FC Schalke 04. He found it hard to make the breakthrough with the Royal Blues, and after a mere seven Bundesliga appearances he moved to Belgium in 2004.
Sint-Truiden
He joined Sint-Truidense V.V. in 2004. In the 2004–05 Belgian First Division season he scored six goals in 29 matches. In the next season he played 20 matches and scored five goals.
1.FC Kaiserslautern
In 2006, Hajnal moved to the Bundesliga to join 1. FC Kaiserslautern. He played 32 matches and scored seven goals in the 2006–07 2. Bundesliga season.
In July 2008, he moved to Borussia Dortmund for a fixed transfer fee of €1.3m. His performance in Dortmund's 4–0 win against Frankfurt on 15 November 2008—scoring once and assists for three goals—earned him a place in the Bundesliga Team of the Week and made Hajnal Man of The Match. In the next round, Hajnal was again in the Team of the Week. He played 30 matches and scored five goals during the 2008–09 Bundesliga season. In the 2009–10 Bundesliga season he played 21 matches. He was injured in 2010, therefore he could not play in the 2010–11 Bundesliga season for Borussia Dortmund.
On 31 January 2011, Hajnal was loaned out to VfB Stuttgart. He scored his first goal against Eintracht Frankfurt on 27 February 2011. The final result was 2–0 for Stuttgart. On 7 May 2011, Hajnal moved permanently to VfB Stuttgart because the club avoided relegation. Hajnal extended his contract with VfB Stuttgart on 7 January 2013 until June 2014 with an option for a further year.
On 1 August 2013, he moved to FC Ingolstadt 04. He made his debut in the 2. Bundesliga against Karlsruher SC. In September 2014 he announced he would be leaving the club, and likely returning to his native Hungary.
Ferencváros
On 12 December 2014, Hajnal returned to his former club Ferencvárosi TC after 17 years. Current manager of Ferencváros, Thomas Doll had been interested in signing Hajnal previously. However, Hajnal was unaware that Ferencváros intended to sign him a couple of weeks before the offer was made. When Hajnal was a free agent, he was training with Ferencváros. Therefore, it was obvious that he would join the club if he would return to Hungary. On 7 March 2015, Hajnal scored his first goal in the 57th minute in the 2014–15 season of the Hungarian League against Győr at the Groupama Arena in Budapest.
International career
On 9 October 2004, Hajnal played his first match for Hungary in Solna against Sweden and on 17 October 2007, he scored his first international goal in a friendly match against Poland. The final result was 1–0 to Hungary. Hungary finished a disappointing sixth in their qualifying group for UEFA Euro 2008 and have been out of the running for a place in Austria and Switzerland next year since the beginning of the qualification period. The competition from Greece, Norway and Turkey was simply too strong for Hajnal and his team mates, who nevertheless beat Bosnia-Herzegovinahome and away during the campaign. Hajnal scored twice against Montenegro in the Puskás Ferenc Stadium in a friendly match. The final result was 3–3. During the FIFA World Cup 2010 qualifying he scored against Malta in Budapest. The final result was 3–0. This goal was the first goal scored in a qualifier. During the management of Sándor Egervári, Hajnal played his first match in the Wembley Stadium against England. Hajnal scored a goal against Azerbaijan on 9 February 2011. Hungary defeated Azerbaijan 2–0 in a friendly match.
