Reiner joined the group in 1995 as CTS automotive engineer, prior to becoming key account manager automotive from 1998 to 2002. He then moved to Brazil, participating in the start-up of Vega do Sul, where he was in charge of supply chain and quality. In 2005 he returned to Europe, to ArcelorMittal Bremen, where he held various positions before being promoted chief operating officer finishing in 2008. From 2011, Reiner was chief marketing officer Business Division South West of ArcelorMittal Europe Flat Products and from 2014 he was chief marketing officer Business Division North of ArcelorMittal Europe Flat Products. In 2017, Reiner became chief executive officer Cluster Bremen, in addition to which he also took up the role of chief executive officer Cluster Eisenhüttenstadt, as from May 2019. He was nominated Vice President of ArcelorMittal in October 2020. Reiner has been in his current role since 1 February 2024.

Reiner graduated as an engineer in production technologies from the Friedrich Alexander University, Erlangen-Nürnberg, Germany.