John grew up in Wythenshawe district of Manchester, where he attended St. Paul's RC High School. He graduated from Loreto College in Hulme and the Manchester Metropolitan School Theatre.
John's first appearance on television was in the TV drama Borgia where he played a minor role as Cardinal Giovanni de' Medici.
John also plays the major character Samwell Tarly in the HBO fantasy TV series Game of Thrones. His character is a friend of Kit Harington's Jon Snow and provides occasional comic relief. One reviewer called him "a wonderful comedic and cowardly yin to Jon's dour yang."