Onslow-Cole began karting at the age of eight and progressed steadily through junior single-seater and saloon car categories. He won the 2006 Renault Clio Cup championship by 113 points, a dominant margin that earned him a step up to the BTCC for the 2007 season with West Surrey Racing — the same organization running under the eBay Motors banner. He finished tenth in the Drivers' Championship that year, recording four podiums including his maiden victory at Snetterton.
For 2008, Vauxhall signed Onslow-Cole to drive a third Vectra alongside champions Fabrizio Giovanardi and Matt Neal. He demonstrated immediate pace, taking pole at Donington Park and claiming his first two BTCC wins in back-to-back races at Thruxton. He closed the year sixth in the championship on 170 points, with two wins and nine podiums — including five consecutive podium finishes at one stage of the season.
After losing his Vauxhall seat to Andrew Jordan, Onslow-Cole spent part of 2009 driving the BTCC medical car before being called upon mid-season by Team Aon to replace Alan Morrison. He signed full-time with the team for 2010, where Aon's LPG-powered Fords showed strong straight-line speed and claimed four of the season's first five pole positions. His most notable result came at Brands Hatch, where he was declared winner after Tom Chilton's exclusion for a technical infringement — holding off Gordon Shedden by 0.009 seconds, recorded as the closest finish in BTCC history.
The 2010 campaign was complicated by team politics: Chilton's father managed the team's Aon Corporation sponsorship, and Onslow-Cole moved aside to allow Chilton victories on more than one occasion, though he was permitted to take a win of his own in the third Silverstone race. In 2011 he moved briefly to AmD Milltek Racing in an NGTC-specification Volkswagen Golf, but parted company mid-season after Oulton Park. He returned to Team Aon for the remainder of the year in a Global Ford Focus.
With Team Aon departing for the World Touring Car Championship, Onslow-Cole joined eBay Motors for 2012, the squad with which he had made his BTCC debut in 2007. He scored a podium on his first race back and collected six further podiums across the season, finishing sixth in the Drivers' Standings. For 2013 he signed a three-year deal with Team HARD. in an NGTC Volkswagen CC, taking his best result of second at Thruxton before missing the final three rounds, at which point he stood tenth in the championship. He finished the year with Motorbase Performance under the Airwaves Racing banner for the final three rounds, achieving a best result of fourth.
Onslow-Cole left touring cars after 2013. He partnered Paul White for the 2014 British GT Championship with Strata21, initially in a Nissan GT-R Nismo GT3 before switching to an Aston Martin Vantage GT3. The pairing completed every round and scored a podium — third at Snetterton — in their first season together.
In 2015 he joined Ram Racing for the 24H Series in a Mercedes-Benz SLS AMG GT3, taking second places at the Dubai 24 Hours and the 12 Hours of Mugello before winning the inaugural 24 Hours of Paul Ricard. Ram Racing secured the GT3 class title and Onslow-Cole earned his first outright GT championship. That same year he won the MSA RX Talent Search at Lydden Hill, which secured him a rallycross outing in the RX Lites support race at the World RX of Turkey; he finished third on his rallycross debut. He also won the Aston Martin festival at Le Mans in 2015.
For 2016, Onslow-Cole raced for Team Parker Racing in a Bentley Continental GT3 in selected rounds of the Blancpain Endurance Series Pro-Am class, while also contesting 24H Series events in Ferrari 488 GT3 and Mercedes AMG GT3 machinery. On 16 October 2016 he won the 24 Hours of Brno driving a Ferrari 488 GT3 for Scuderia Praha.
In 2017 he made his debut as a BMW factory driver in the Nurburgring 24 Hours in the M6 GT3, preparing with VLN rounds in advance. He also contested the Le Mans Cup with Ram Racing and GT Open rounds with sps automotive performance, both in the Mercedes AMG GT3.
His GT career continued to yield results at the elite Pro-Am level, culminating in back-to-back International GT Open Pro-Am championship titles in 2018 and 2019.
Onslow-Cole's BTCC career spanned seven seasons and produced moments that remain in the record books, most notably the 0.009-second margin at Brands Hatch in 2010 — still cited as the closest finish in the championship's history. His subsequent pivot to endurance and GT racing demonstrated adaptability, with championship wins at multiple levels of the 24H Series and International GT Open confirming him as one of the more versatile British drivers of his generation.