After starting his career in lower motor sport categories, he joined Toleman in the early 1980s. As Toleman grew, it was taken over to become Benetton Formula, and was subsequently sold and renamed Renault F1. Symonds remained throughout this entire period with the team, working his way through the technical ranks. Symonds served as an engineer for many of the team's drivers, including Alessandro Nannini and Teo Fabi.
After a brief move to the abortive Reynard F1 project with then-chief designer Rory Byrne in 1991, in the mid-1990s he was Michael Schumacher's race engineer while also assuming the role of Head of Research and Development. Symonds remained with the team when Schumacher departed to Ferrari in 1996. When Ross Brawn was also lured to Ferrari, Symonds became the team's Technical Director. When Mike Gascoyne joined Benetton in 2001, Symonds was promoted to Executive Director of Engineering, a post which he retained though the transition to Renault ownership in 2002.
The ING Renault F1 Team released a statement on 16 September 2009 stating that Symonds was no longer part of the team. Symonds was subsequently suspended from F1 events for 5 years after expressing his “eternal regret and shame” to the FIA World Motor Sport Council. However, his ban was overturned by the French Tribunal de Grande Instance on 5 January 2010, and he was also paid €5,000 in compensation. In April, he and Briatore reached an out-of-court settlement with the FIA where he could return to F1 in 2013 but may be a consultant to a current Formula 1 team in the meantime.
In 2011, Symonds returned to F1 as a consultant for the Virgin Racing (now Marussia F1) team in order to conduct a thorough overview of its operation, following a disappointing start to its second season in the sport. Shortly afterwards, the team parted company with existing technical director Nick Wirth. Symonds is believed by many to have effectively taken Wirth's place, although he was still only a consultant due to the terms of his ban.
Symonds had a column in the F1 Racing magazine, and serves on the committee for the MSc in Motorsport Engineering and Management at Cranfield University.
In July 2013 it was announced that Symonds has been appointed as Chief Technical Officer to the Williams F1 Team, replacing the immediately departed Mike Coughlan.