Whilst at university, Klingbeil worked in the constituency offices of Chancellor Gerhard Schröder and Heino Wiese [de] from 2001 until 2003.

After finishing his studies he worked as youth education advisor for the Social Democratic Party in North Rhine-Westphalia.

Leaving office after the 2005 national elections, Klingbeil worked as chief of staff to the chairman of the Social Democratic Party in Lower Saxony Garrelt Duin.

On 19 October 2017, Social Democratic Party chairman Martin Schulz nominated Klingbeil as secretary general.

In the negotiations to form a fourth cabinet under Chancellor Angela Merkel following the elections, he was part of the leadership team of his party's delegation.