From 1979 to 1990 he was employed by Ford, working between Dunton, Japan, Italy and Australia, mainly on "bits of cars, mostly steering wheels". As well as working on bread-and-butter Fiestas and Mondeos, he contributed to image builders such as the RS200 and Escort RS Cosworth, the last of which he is especially proud of and with which he collaborated with fellow RCA graduate Peter Horbury. He was then appointed Design Manager responsible for the Ghia Design Studio in Turin, where he worked on the Via, Zig and Zag show car concepts.
TWR
After eleven years in a corporate environment, Callum left Ford in 1990 to join Peter Stevens and Tom Walkinshaw to form TWR Design. He said, In 1991 he was appointed Chief Designer and General Manager of TWR Design. During this period he was partially responsible for designing the Aston Martin DB7, which is probably the design he is currently most famous for. He also designed the Aston Martin Vanquish, the V12-powered DB7Vantage and Aston Martin's Project Vantage concept car as well as taking responsibility for a wide range of design programs for other TWR clients, including Volvo, Mazda and HSV. He was awarded the Jim Clark Memorial Award in 1995 in recognition of his styling work on the DB7. In 1998, he designed the Nissan R390.
Jaguar
In 1999, on the death of Geoff Lawson, Callum was appointed to succeed him at Jaguar, which was a Ford Motor Company subsidiary at the time. For a short stint, Callum directed design at both Jaguar and Aston Martin and was responsible for the majority of the DB9 and Vantage, both of which are attributed to Henrik Fisker. At Jaguar, since the Lawson designed 2001 X-Type and 2002 XJ were well advanced his influence was initially felt through a series of concepts, the 2001 R-Coupe and 2003 R-D6. The first production Jaguar to bear his influence was the 2004 facelift of the S-Type followed by the 2004 X-Type Estate, of which he oversaw the tailgate design. With the next generation of Jaguar models Callum took Jaguar away from the Lawson-era retroism, which produced the more traditional-looking X-Type, S-Type, and XJ, towards a new style. This began with the 2006 XK, which bears striking similarity to the Aston Martin DB9, which Callum describes as being a result of modern safety legislation. This new direction continued with the 2008 XF as previewed by the concept C-XF, which Callum describes as the 'next significant step forward' in Jaguar's design direction, and the 2010 XJ. He also oversaw the design for the 2010 C-X75 concept car, the 2012 refresh of the Jaguar XF and introduction of Sportbrake, the 2013 F-Type, the 2015 Jaguar XE, 2015 second generation Jaguar XF, the 2016 Jaguar F-Pace, and the 2018 Jaguar I-Pace. According to Callum, ‘Jaguars should be perceived as cool cars and cool cars attract interesting, edgy people.’ The quality of Callum's work at Jaguar has been acknowledged by twice Le Mans 24 hours winner and fellow Dumfries native, Allan McNish. In early June 2019, Callum announced that he was stepping down to "pursue other design projects" but would continue with the company as a design consultant. "Designing Jaguar cars was a lifelong dream for me, and I’m delighted to remain involved as a consultant for the brand", he said in an interview. Callum was appointed Commander of the Order of the British Empire in the 2019 Birthday Honours for services to the British car industry.
