[2] His parents were the proprietors of a women's clothing chain in Wales and his maternal grandfather was the Belgian oil industrialist Edmond Jules Dupont from Liège.

[3] Mike Gwilym's interest in acting began while at Wycliffe Preparatory School,[4][5] but he began his acting career while at university at Oxford with the Oxford University Dramatic Society, and went on to join the Glasgow Citizens' Theatre before becoming an associate actor of the Royal Shakespeare Company His stage debut was as 'Prince Hal' in Henry IV, Part 1 at the Playhouse Theatre, Sheffield, UK in 1969.

[2] He starred in many of their productions during the late 1970s and early 1980s, including The Comedy of Errors, King Lear, Troilus and Cressida, and Love's Labour's Lost.

In the BBC Television Shakespeare series, he starred in Coriolanus (as Aufidius), in Love's Labour's Lost (as Berowne), and Pericles, Prince of Tyre in the title role.

Gwilym retired from the professional stage to the South of Spain (province of Malaga), where his parents had a summer home.