Simon Thomas was born in Norwich, Norfolk to Andrew and Gill Thomas. He has two sisters called Hannah and Rebecca. Although Thomas initially grew up in Cromer, Norfolk and then Grimston, Norfolk where he attended Grimston primary and middle School, the family later moved to Surrey, where he attended Aberdour School in Tadworth, and St. John's School, Leatherhead. After leaving school, he went to Birmingham University, where he graduated with a 2:1 BA Hons degree in history.
Having applied for the job three times, Thomas began presenting the children's programme Blue Peter on 8 January 1999 replacing the sacked presenter Richard Bacon. Previously he had worked as a runner at Children's BBC and the radio station LBC. During his tenure at Blue Peter, he ran in the London Marathon twice and two Great North Runs, climbed Mount Kilimanjaro and Mont Blanc too, completed over 40 solo sky dives with the Royal Air Force, presented a medal at the Commonwealth Games, ran in the 60 metres sprint at the Norwich Union Indoor Athletics against Frankie Fredericks and Darren Campbell and even directed an edition of the show. He also presented the Queen with a Gold Blue Peter badge when she visited the studio and remarked on presenting Her Majesty with the badge that "she could get into the Tower of London free with it".
Thomas co-presented with Stuart Miles, Katy Hill, Konnie Huq, Matt Baker, Liz Barker and Zöe Salmon.
Thomas announced his intention to leave Blue Peter in January 2005, and his last live show was aired on 25 April of that year.
Thomas presented CBBC Proms in the Park in Hyde Park in 2002 with Matt Baker and Faye Tozer from Steps; and in 2003 with Fearne Cotton. He also presented BBC Radio 3's Making Tracks with Matt Baker.
After leaving the BBC, Thomas joined Sky Sports in June 2005 and presented the 3-7pm shift on Sky Sports News, alongside Georgie Thompson. He also presented Cricket AM during the summer on Sky Sports 1 with Sarah-Jane Mee and has presented the Sky Sports Victory Shield, England's U21's games, Centenary Shield, International Cricket, Sky Sports News Cricket World Cup Report, the Mosconi Cup from Las Vegas, Sky Sports World Cup report from Cape Town 2010 and Sky Sports News's coverage of the 2010 Ryder Cup.
At the start of the 2010/2011 Football Season, Thomas became the lead presenter of Sky Sports' live Football League coverage, leaving his position on Sky Sports News. From 2014, Thomas began presenting live League Cup football on Sky Sports, following Ben Shephard stepping down from all live match presenting duties. From August 2016, Thomas has presented live Premier League coverage, most notably on Saturday lunch times and bank holidays, following the departure of Ed Chamberlin to ITV Racing, and David Jones' subsequent promotion to Super Sunday and Monday Night Football lead presenter. Thomas regularly works alongside pundits such as Graeme Souness, Jamie Redknapp and Thierry Henry.
In August 2009 and August 2010, Simon compered on the Main Stage at Greenbelt, a Christian music and arts Festival held at Cheltenham Racecourse.
Simon lives in Reading and is married to Gemma Thomas and has a son called Ethan. He is a lifelong supporter of Norwich City FC and Vice-President of the Norwich City Supporters Trust. He regularly attends and speaks at Greyfriars Church, Reading.