Dietmar or simply "Didi" grew up in Bavaria and was snapped up by Bayern from Wacker Munich when he was 16 years old. He enjoyed tremendous success with Bayern, winning two Bundesliga titles, the UEFA Cup, the German cup and the German league cup. He played 106 Bundesliga games, scoring six goals for Bayern before testing himself in English football. After starring for Germany in the 1998 World Cup he was transferred to Newcastle United for £5.5million. Hamann had a single season at Newcastle playing 31 League and cup matches and scoring five goals, losing an FA Cup final against Manchester United and getting himself sent off in a fiery Premier League match against his future employers Liverpool at Anfield. A goal up at the time of his dismissal, Newcastle increased their lead before Liverpool hit back with four second-half goals of their own. Hamann was one of several new signings unveiled by Gerard Houllier during the summer of 1999. He made his League debut for the Reds at Sheffield Wednesday on the opening day of the 1999/00 season, as did Titi Camara, Sami Hyypia, Vladimir Smicer, Sander Westerveld and Eric Meijer. The German's debut lasted only 24 minutes as he ruptured his ankle ligaments. Fortunately he healed quickly, was back in seven weeks and represented Liverpool in another 29 matches by the end of the season.

Didi missed very few matches over the next three seasons as he cemented his place in the team as a defensive midfielder with an occasional eye for a beautiful goal. He could always be relied upon, although Jamie Carragher, who was one of his many friends at the club, nominated him as the worst trainer. "Didi! He's a joke. He shouldn't be at the club, should he?" Carra quipped. In 2000/01 Hamann won his first winners' medals in English football by appearing in all three of Liverpool's successful cup finals. A second League Cup winners' medal would follow in 2003, that, in fact, he wanted to give to one of the younger players who had appeared in earlier rounds. Didi explained: "Valuing everyone who had made an effort for us was more important to me than personal gratification.” It was soon apparent that new manager Rafa Benítez saw the tall German as an equally important member of the squad as Houllier had done. Hamann played in 43 of Liverpool's 60 competitive matches in 2004/05 but probably his most important contribution came in the final game of the season, that astonishing Champions League final in Istanbul. His half-time substitution for Steve Finnan was one of the catalysts for Liverpool's truly amazing comeback. He also proved typically cool when scoring a penalty in the shoot-out, later discovering that he actually took that kick with a fractured foot! A year later he would again successfully cope with the same pressure when the FA Cup final against West Ham in Cardiff was settled in the same way. But 2005/06 was a season in which Hamann's influence was waning as Momo Sissoko's influence grew. At the age of 32 he was told by Benítez that he should expect not featuring a lot for the first team and he signed a contract with Bolton Wanderers in the summer of 2006. He changed his mind but it was too late to cancel his transfer to Bolton and in one of the most bizarre transfers ever he moved from Bolton to Manchester City 24 hours later for £400,000. Probably the most expensive "free transfer" in history! Hamann explained: "I had the offer from Bolton for a few weeks and thought it was the right thing to do. But after going on holiday I felt it wasn't the right decision. It is something I have got to live with and they allowed me to speak to other clubs. As soon as I heard Man City was interested I made my mind up pretty quickly." 

Hamann had three seasons at Eastlands, during which he continued to show that his ability to perform at a high level was undiminished. 54 Premier League and 17 cup appearances for City took his combined club total in Germany and England past the five hundred mark. After a year out of the game, Hamann signed a one-year deal on 20 May 2010 to become player-coach with the Milton Keynes Dons in League One. On 3 February 2011 the German left the Dons to join Leicester City as a first-team coach under the management of Sven-Göran Eriksson. Five months later Hamann became manager of Stockport County where he faced a massive task trying to lead the club immediately back into the Football League. Unfortunately, Stockport made a very poor start to the 2011/12 Conference season and with the team having won just three of its opening 19 League fixtures, Hamann resigned from his post on 7 November 2011. 

In 2012 Hamann published his autobiography The Didi Man: My Love Affair With Liverpool and began doing punditry for the BBC and Sky Sports. He has also been a studio analyser for Irish channel RTE at the European Championships of 2012 and 2016, the World Cups of 2014 and Champions League in 2015-16 and 2016-17. He has not been involved in management since, bit did have a brief spell as a player again with German amateur side TuS Haltern towards the end of 2014-15.

"When I played for Liverpool they said I was the only man who could run faster going backwards than forwards." A gem from Didi who was immensely popular among the Liverpool players, becoming an adopted Scouser in his seven years at the club. He liked a flutter on the horses and tried to impress on the dance floor as Chris Kirkland noted: "Didi fancies himself as a bit of a dancer, which is a shame because when you've seen him dance it's not a pretty sight! Didi likes his karaoke too and he's not usually too far behind Carra grabbing the microphone. Didi's a great laugh." Steven Gerrard has plenty of praise for his midfield partnership with Hamann: "He allowed me to go forward, score and set up goals, and he did all my defensive work and tackling for me," Stevie enthused. "At the time we had him, he was in his prime and he was up there with the best holding midfielders in the world. There’s no Liverpool fan, player or coach at Anfield who wouldn’t love him to be 26 or 27 now and still doing his stuff because that’s how good he was."