Between 1991 and 1995, Future Publishing employed him and he designed and painted more than 200 magazine covers.
In 1995 he started working exclusively for Terry Pratchett’s Discworld series. His Discworld art, and many descriptions of the pieces, are included in The Pratchett Portfolio (1996) and The Art of Discworld (2004). These showcase full versions of work and some paintings previously unseen. He also illustrated The Last Hero, a Discworld "fable" and number-one bestseller in the United Kingdom, his "90 beautiful illustrations" described as "augment[ing] the impact of this vividly described magical world."
Kidby's art is in several cases a tribute to (or a parody of) well known paintings such as the Mona Lisa by Leonardo da Vinci,Night Watch by Rembrandt (featured on the cover of Night Watch) and An Experiment on a Bird in the Air Pump by Joseph Wright.
In December 2007 Terry Pratchett used Kidby's website to announce he was suffering from early-onset Alzheimer's disease.
In August 2016 Kidby released "Terry Pratchett’s Discworld Colouring Book" of black and white line drawings. He had been designing the cover of the book in March 2016 when he heard that Pratchett had died.