Johnson joined the ITV Wales news programme Wales Tonight as a reporter in January 2009 and also presented The Wales Show until December 2010.
In September 2010, Johnson joined the interactive "Hub" team on ITV's This Morning programme. On 3 June and 28 October 2011, he guest co-presented the main This Morning show on ITV. Johnson left This Morning in 2013 and presented his final show on 30 August.
From 14 February to 16 August 2011 Johnson was co-presenter of Channel 5's OK! TV. In 2012, Johnson appeared as a celebrity contestant on the Christmas edition of Take Me Out. In August 2013, he was a contestant on an episode of Tipping Point: Lucky Stars He appeared as a contestant on a celebrity episode of The Chase in December 2014.
Johnson occasionally presented The National Lottery Draws on BBC One. He also co-presented Hwb, a programme for Welsh learners, on Welsh-language channel S4C. In 2015, he became a reporter for the ITV series Surprise Surprise. In October 2016, Johnson took part in Celebrity Haunted Hotel Live, presented by Christine Bleakley.
In 2013, Johnson was a contestant on the ITV series Your Face Sounds Familiar, performing as different singers each week. Johnson finished as one of the runners-up to winner Natalie Anderson.
In 2007, Johnson appeared on an episode of Gavin and Stacey playing Jose, Stacey's brother Jason's boyfriend.
Johnson is an ambassador for the mental health charity Mind. In May 2017 he presented Matt Johnson: Iselder a Fi (Depression and Me) on S4C, a Welsh-language personal documentary about issues of mental health, especially among young men, including his own story of struggling with depression.
He is an ambassador for the RSPCA animal charity.
In 2010, Johnson was voted Wales' Sexiest Man.
In 2013, Johnson was voted Most Entertaining Person on Channel 5's Big Brother's Bit on the Side for the episode broadcast on 12 September.
In 2015 Johnson was in the top 50 most in-demand charity ambassadors in the world,