Portman is the son of actor Ron Donachie and Fiona Biggar, and has a sister Naomi.

He attended Shawlands Academy, where he played rugby and was named Head Boy.

[1] He then pursued a Higher National Certificate (HNC) in Acting at Reid Kerr College in Paisley.

It was announced on 24 August 2011 that he was cast as Podrick Payne in the award-winning HBO fantasy drama series Game of Thrones.

[5][6] He contributed vocals to a version of the track "Jenny's Song" that featured in the episode "A Knight of the Seven Kingdoms".