He is best known as T-Shirt in the children's television series T-Bag from 1985 to 1992, and for voicing Thomas the Tank Engine in the UK dub of the children's television series Thomas & Friends from 2015 to 2021, Arnold McKinley and James Jones in Fireman Sam from 2012.
Hasler was the only cast member on T-Bag to remain with the show for its entire television run, starting as a young boy and leaving as a teenager.
He has appeared in Renford Rejects and Casualty and provided voices and additional dialogue replacement (ADR) for a number of film, radio and television productions.
In 2012, he joined the voice cast of Fireman Sam as James Jones, taking over the role from Steven Kynman and Arnold McKinley.
[2] From August 2012 to February 2015, Hasler was cast in the United Kingdom tour of Disney's The Lion King, performing as the meerkat Timon.