His notable roles include the films What Richard Did (2012), Transformers: Age of Extinction (2014), Glassland (2014), Macbeth (2015), Sing Street (2016) and Midsommar (2019), as well as the series Strange Angel (2018–2019) and The Peripheral (2022).

[2] He initially lived in Boulder, Colorado, with his mother, human rights activist Tara Reynor O'Grady,[3] but moved with her to Valleymount, County Wicklow, at the age of two.

In early 2010, Reynor was cast as Robbie, "the boy next door", in the live-format, unscripted Dollhouse, directed by Kirsten Sheridan, which premiered at Berlinale 2012.

The film portrays the fall of a high school rugby star and golden boy whose world unravels after his involvement in an act of careless violence.

Also that year, Reynor starred in writer-director Gerard Barrett's film Glassland with Toni Collette and Will Poulter, in which he plays a young taxi driver who struggles to instil a sense of structure in his life while his mother suffers from severe alcoholism.

[11] His role opposite Sarah Gadon was the character RAF airman Jack, the young Princess Elizabeth's fictional romantic interest during the VE Day celebrations in 1945.