Kádár made his debut for Zalaegerszegi TE at the age of 16 and has since gone on to win Hungary U-21 honours.
He was offered trials by Premier League clubs Bolton Wanderers and Newcastle United in December 2007 and arrived at St James' Park on a full-time deal.
Kádár's transfer was allowed due to Hungary being within the European Union. Normally a player must be 18 years old at minimum, but within the European Union players could be transferred at the age of 16.
Kádár became the 3rd youngster to sign for Newcastle in the January transfer window following the signings of Ben Tozer and Wesley Ngo Baheng.
After some hold-ups as a result of Sam Allardyce's sacking, the deal was confirmed on 18 January, the same day that Kevin Keegan was unveiled to the press for his second stint as manager.
During the 2008–09 season, Kádár was mostly plying his trade in the reserves but appeared on the bench as an unused substitute for Newcastle's home league match with Liverpool and the FA Cup 3rd round game with Hull City.
On 21 January 2009 he suffered a broken leg in a reserve match against Sunderland, ruling him out for six months.
On 11 July 2009 he returned to action for Newcastle United, helping in their victory 3–0 over Shamrock Rovers. This was Newcastle's first pre-season match of the 2009–10 season.
He made his full first team debut in the 4–3 victory over Huddersfield Town in the League Cup in August, before making his league debut on 31 August as part of a 1–0 home win against Leicester City.
Although behind Steven Taylor and Fabricio Coloccini in the pecking order, he has turned in some strong performances whenever new manager Chris Hughton played him, especially when Taylor and Coloccini were out injured during various times of the season.
In the 2010-2011 season, he was given a chance in the opening rounds of the League Cup playing in central defence for their round two clash with minnows Accrington Stanley. Newcastle won 3-2 thanks to goals from Peter Lovenkrands, Shola Ameobi and Ryan Taylor. In the 2011–12 season of the Premier League Kádár became close to the first squad after the injuries of Fabricio Coloccini and Steven Taylor.[
However, he was released by Newcastle United on 1 June 2012.PRE-SEASON FIXTURES:
DEN HAAG 0 NEWCASTLE 0Krul; S Taylor (Gosling 65), R Taylor (Perch 71), Coloccini, Santon (Simpson 80); Obertan (Bigirimana 71), Tiote (Ameobi 71), Vuckic (Williamson 65), Gutierrez; Ba (Amalfitano 82), Cisse
BRAGA 1 NEWCASTLE 2 (Ba, R Taylor)Harper; Simpson, Tavernier, Coloccini, R.Taylor; Sammy Ameobi (Obertan 88), Tiote (Cabaye 70), Gutierrez, Marveaux; Cisse, Ba (Vuckic 85).
OLYMPIACOS 1 NEWCASTLE 1 (S Taylor)Krul, Perch, Williamson, S.Taylor (Dummett 46), Santon, Obertan (Sammy Ameobi 72), Bigirimana, Gosling (Abeid 36), Amalfitano, Vuckic, Shola Ameobi (Cisse 72).
FENERBAHCE 1 NEWCASTLE 1 (Abeid)Elliot, Simpson (Santon 68), Tavernier, Coloccini, R Taylor, Tiote (Bigirimana 59), Gosling (Abeid 73), Obertan (Shola Ameobi 75), Marveuax (Gutierrez 59), Vuckic (Sammy Ameobi 46), Ba (Cisse 68).
MONACO 0 NEWCASTLE 1 (Ba)Harper, Santon (Ryan Taylor 57), Simpson, Williamson (Perch 46), Gutierrez, Amalfitano (Obertan 62), Abeid (Tavernier 46), Bigirimana (Tiote 62), Sammy Ameobi (Vuckic 46), Shola Ameobi (Gosling 62), Ba (Marveaux 78)
CHEMNITZER 1 NEWCASTLE 0Elliot; Tavernier (Simpson, 70), Williamson, Perch, R Taylor (Santon, 70); Obertan (Gutierrez, 77), Abeid (Bigirimana, 70), Gosling, Amalfitano; Vuckic (Shola Ameobi, 70), Sammy Ameobi (Ba, 77). Sub not used: Harper. Att: 6,700
Sat 11.08.2012 Cardiff City v NUFC (at Cardiff City Stadium, Wales - 3pm KO)