Reith was born in Melton, Suffolk. He studied acting at the Webber Douglas Academy of Dramatic Art. He began acting in the late 1970s, beginning with an appearance in International Velvet (1978) alongside Tatum O'Neal and Christopher Plummer.
He worked as an announcer and presenter at BBC Radio 3 for five years before leaving to study Greats at Christ Church, Oxford for four years beginning in 1989. He worked as an teacher, including at Westminster School, before resuming his acting career.
Reith joined the cast of Downton Abbey as Lord Merton in 2012, and alongside the cast was nominated for the Screen Actors Guild Award for Outstanding Performance by an Ensemble in a Drama Series at the 23rd Screen Actors Guild Awards. He reprised the role in the films Downton Abbey (2019) and Downton Abbey: A New Era (2022). In 2017, Reith was cast in Tim Burton's live-action remake Dumbo (2019). In 2019, he was cast in SAS: Red Notice (2021).

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