Hasler began his career in the mid-1980s. He played T-Shirt, a prominent character in the T-Bag TV series, between 1985 and 1992, being the only character to star in all nine series and in all four Christmas specials.
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. He also appeared for an old TV advert for Persil washing powder in the 1990s.
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.3
In 2015, he became the replacement voice actor of Thomas in the UK and Rheneas in the UK and US narrations of Thomas & Friends, replacing Ben Small after he left the show, until the series ended in 2021, however Hasler continues to voice him in Youtube shorts.4 He also voices the lead character Trafalgar on Teenage Fairytale Dropouts, protagonist "Toot" in Toot the Tiny Tugboat and Foz in Go Jetters.
In 2021, Hasler provided the British English voice of Hugo the Jungle Animal in all three films.
"John Hasler – 5 Character Images". Behind The Voice Actors. https://www.behindthevoiceactors.com/John-Hasler/ ↩
"John Hasler – Actor and Voiceover Artist". www.johnhasler.co.uk. Retrieved 27 June 2019. http://www.johnhasler.co.uk/ ↩
"The Lion King Roars Off On First-Ever U.K. Tour". Encore Tickets. Retrieved 27 June 2019. https://www.encoretickets.co.uk/news/the-lion-king-roars-off-on-first-ever-uk-tour ↩