Boateng was with Hertha Berlin from July 1, 2002 when he was 13 years old, to 22 August 2007. After emerging from the youth team set-up, he played for the reserves for one season, before winning a call-up to the first team squad on 31 January, 2007. He made his debut against Hannover 96 in the AWD-Arena, during week 19 of the 2006-07 Bundesliga season. He then became a first team regular, despite being barely 18 years of age.
Boateng has played several times for the German Under 16, Under 17 and Under 19 national teams. On 5 July, 2007 he was called up to the German under-19 squad by coach Frank Engel for the 2007 UEFA European Under-19 Football Championship held in Austria, from July 16 until July 27.
Boateng was linked with a move to Hamburger SV in the summer of 2007. Because of this, according to the German media, Boateng did not want to sign a five year professional contract with Hertha, nor extend his amateur contract, as he wanted his salary to be increased from €100k to €1 million per year. He moved to Hamburg on 22 August 2007 for a fee said to be around €1.1M.