His career has included stints with Network Ten in Australia and the BBC in the United Kingdom covering various forms of motorsport.

In the United States, Diffey joined Speed Channel in 2003 before moving to NBC when Formula One's US television rights were transferred to the network in 2013.

[3] Meanwhile, he began working in motorsports broadcasting; his first job came in 1990 at the age of 19, calling Ipswich Motorcycle Club racing at Tivoli Raceway.

[4] After two years with the Australian Super Touring Championship broadcasting team for Network Ten, Diffey became the V8 Supercars lead announcer in 1997, partnering with Greg Rust,[2] Barry Sheene, Bill Woods and Mark Oastler.

[20] In December 2015, Diffey, Hobbs and Matchett were given an Honorable Mention in a list of Best Broadcast Teams of the year published on the Sports Illustrated website SI.com.

On 23 August 2015, he called the Belgian Grand Prix from NBC's base in Stamford, Connecticut, before he and Matchett traveled to Pocono Raceway for the ABC Supply 500.

[23] Diffey's open-wheel focus shifted solely to IndyCar in 2018 when the US Formula One television rights were transferred to ESPN and they chose to take commentary from Sky Sports insteading of making their own.

[34] In August 2017, Diffey served as the lead announcer for NASCAR on NBC's primary coverage of the renamed Monster Energy Cup Series for two races (the I Love New York 355 at The Glen and the Pure Michigan 400), filling in for regular announcer Rick Allen who was working in London with NBC's coverage of the 2017 IAAF World Championships.

[35] On 13 March 2024, it was reported that Diffey would replace Allen in NBC's NASCAR Cup Series booth shortly after the 2024 Summer Olympics.

[40] Diffey began calling the AMA Supercross Championship in 2020, working play-by-play for NBC for the series along with Ralph Sheheen.

[41] Diffey took over the lead commentating role replacing Sheheen starting the 2021 season with Ricky Carmichael, Daniel Blair, and Will Christien.

[46] NBC executive Sam Flood had expressed interest in having him serve as an announcer outside of motorsports while negotiating his contract with the network,[47] and he prepared Diffey for the Olympics by having him host the Penn Relays and the Luge World Cup.

[54] At the 2024 Summer Olympics in Paris, Diffey botched the call of the men's 100 metre final, hastily declaring Jamaican Kishane Thompson the winner in a photo finish which found American sprinter Noah Lyles officially winning the gold medal.

[55] Diffey took responsibility for the mistake in a social media post the following day, saying, "My eyes [and] instinct told me Kishane Thompson won.