Stephen was first elected as the Member of Parliament for the Aberavon constituency in 2015.  He was subsequently re-elected to represent Aberavon in 2017 and 2019.  Following the boundary changes that saw Aberavon become the new Aberafan Maesteg constituency, Stephen was again successfully elected in 2024.

In Opposition, Stephen has served as Shadow Asia and Pacific Minister, the Shadow Armed Forces Minister, and Shadow Immigration Minister.

Following the successful General Election in 2024, where Labour won a 174-seat majority, Stephen was asked to serve in HM Government as the Minister of State (Department of Health and Social Care).