[1] After graduating from Ruhr University Bochum in 1989, Geier worked as a research assistant to Social Democrat MEP Detlev Samland.

[2] Geier then spent five years working for Projekt Ruhr GmbH, a regional development agency, before taking up a position at Deloitte in 2006.

[7] In addition to his committee assignments, Geier has served as a member of the parliament's delegations for relations with the United States and Japan.

[12] In this capacity, he served as the parliament's rapporteur on a non-binding 2021 motion in support of using “low-carbon hydrogen” made from fossil gas as a bridge towards 100% renewable production.

[14][15] In the negotiations to form a coalition government under the leadership of Chancellor Angela Merkel following the 2017 federal elections, Geier was part of the SPD delegation in the working group on European policy, led by Peter Altmaier, Alexander Dobrindt and Martin Schulz.