Newbold was born in Jacksdale, Nottinghamshire, where his parents ran a butcher's shop. He grew up in the village and later, in married life, lived in South Normanton. His introduction to motorcycle riding came at the age of 15, when he began competing on grass tracks. He progressed steadily through club and national competition, developing his craft at Darley Moor on a machine loaned to him by fellow racer John Cooper, before moving on to perform in front of crowds of 50,000 at Silverstone Circuit.
Newbold reached the Grand Prix stage and established himself as a competitive rider in the 500cc class. His finest season came in 1976, when he finished in fifth place in the 500cc world championship aboard a Suzuki. The highlight of that year was his victory at the 500cc Czechoslovak Grand Prix, his only world championship race win. He achieved this as a teammate of Barry Sheene and John Williams within the Suzuki factory setup, a team that was at the forefront of the sport during that period, with Sheene going on to win the 500cc title in 1976 and 1977.
Beyond the Grand Prix calendar, Newbold was a respected competitor on the road racing circuits of the British Isles. He won the North West 200 in 1978, a prestigious event held on public roads in Northern Ireland that has long been a proving ground for top-level road racers.
Newbold made his debut at the Isle of Man TT in 1981, completing the course and finishing in fourth and third place in his two races, a promising start at one of the most demanding events in motorcycle sport. Also in 1981, he was the top points scorer for Great Britain in the Transatlantic Trophy competition, a series pitting British riders against their American counterparts.
On 15 May 1982, Newbold was killed during the North West 200 race in Coleraine, Northern Ireland, after a collision with fellow competitor Mick Grant. His death came at a circuit he had previously conquered in 1978, and it added his name to the list of riders who lost their lives on the challenging public road courses of the British Isles during that era.
John Newbold's career was defined by steady competitiveness in the 500cc class at a time when Suzuki was challenging for world championship honours. His 1976 Czechoslovak Grand Prix victory, achieved alongside some of the sport's leading names, demonstrated his capability at the highest level. His record on the roads, including the North West 200 victory and his Transatlantic Trophy performances, showed the breadth of his talent across different competitive formats. He is remembered as a capable and well-regarded professional from the golden era of two-stroke 500cc racing.