Riddell began his media career with a cadetship at Messenger Newspapers in Adelaide.[2] Riddell then moved into radio where he worked at 5DN and then as a foreign correspondent for the Macquarie Radio Network.[3] While in London covering the assassination of Lord Louis Mountbatten for Macquarie in 1979, Riddell caused a security scare and was ejected from Australia House when he was mistaken for a bomber when he was seen with an unscrewed telephone receiver with its wires connected to a small black cassette recorder as he attempted to send a pre-recorded report back to Australia.[3]
He then moved into television, initially working at TVW-7 in Perth before returning to Adelaide where he started working at NWS-9 in 1981.[3]
After spending nine years at NWS-9, he moved to SAS-7 where he anchored the weekend Seven News bulletins.[2] When Graeme Goodings moved to weekends as he recovered from cancer, Riddell moved to the weeknight bulletin in 2005 where he formed an enduring on-air partnership with Jane Doyle.[4] While at Seven, Riddell also occasionally hosted the Adelaide edition of Today Tonight.[5] He also hosted the documentary series The World Around Us and the local AFL program The Crows Show, despite being an ambassador for Port Adelaide.[3][6]
He also anchored Seven's coverage of South Australian state elections.[7] However in a 2019 interview, he admitted that he would have said no to hosting election coverage if he'd been able to.[3]
Riddell also continued to work in radio. From late 2007 to early 2012, he was a co-host on the Mix 102.3 breakfast show, alongside Jason "Snowy" Carter and Jodie Oddy.[8][9] With Carter, Riddell inaugurated an annual charity fundraiser called the West End Mighty River Run to raise money for Novita Children's Services in 2006.[10] Riddell briefly returned in radio in June 2012 filling in for Dale Lewis on Triple M Adelaide's Hot Breakfast.[11]
After his 63rd birthday in September 2019, Riddell announced his retirement.[1] His co-anchored his final Seven News bulletin on 6 December 2019.[12] After his retirement, Jane Doyle continued as the sole anchor of the bulletin until her own retirement in late 2022.[13]
Following the South Australian bushfires of late 2019 and early 2020 which affected the Adelaide Hills and Kangaroo Island, Riddell returned to Seven co-host a special "Book Them Out" edition of SA Weekender to help support the local communities.[14]