is an English television presenter and mathematician who currently co-presents the Channel 4 daytime puzzle show Countdown, its comedy spin-off 8 Out of 10 Cats Does Countdown and Sky Sports' Friday Night Football.
A mathematics graduate, her television debut came when she joined Countdown at age 22; with a passion for popularising maths and the sciences she has gone on to co-present The Gadget Show on Channel 5 (2013–14) and It's Not Rocket Science on ITV (2016).
While inhabiting the niche role, she long believed she was not a television presenter, but she came to realise that the industry has changed such that "I don't think there's such a thing as a TV presenter any more. As her popularity grew due to daily appearances on Countdown, she branched out into other roles, co-presenting 8 Out of 10 Cats Does Countdown and The Gadget Show, although her profile was significantly boosted by her appearance as a contestant on Strictly Come Dancing, which largely "catapulted [Riley] onto the A-list" and opened up the doors for new television roles, according to another commentator.
On Countdown, Riley replaced the long-serving Carol Vorderman.
Riley takes the role of solving the numbers round so seriously that if she cannot solve it on the show, she takes it away with her to finish, commenting "I’m really OCD about it. I like puzzles – that’s what I do, that’s why I applied for the job, that’s why I used to watch Countdown when I was younger.
I like solving stuff." She has attributed this compulsion to a nervy and flustered start in the role which resulted in her making some errors, although she says she has since got the hang of it and the task is now a matter of routine.
Like Vorderman, Riley's specific role was to handle the placement of letters and numbers in the anagram puzzle and arithmetical problem rounds, in addition to solving the arithmetic problem if the contestants have been unable to do it.
Riley's appointment to Countdown was announced at the same time as the announcement of a new main host, Jeff Stelling, replacing Des O'Connor; the duo of Stelling and Riley were in place for the new series beginning in January 2009.
She then studied mathematics at Oriel College, Oxford, gaining an upper second-class honours degree.
During a university vacation, Riley considered a career in the financial sector, and completed an internship in The City of London for Deutsche Bank.