Newton started his career at second-tier Charlton Athletic and worked his way up to the first team, making his debut as a 17-year-old on 14 August 1993 as a substitute in a 1-0 win over Birmingham City. He quickly became a first-team regular and was a virtual ever-present between 1995 and 1998. This era also saw him recognised by his country as he won three England Under-21 caps during 1996. His debut came in a 2-0 win over Moldova U-21 on 31 August 1996.
He won promotion to the Premier League in 1998 after a momentous play-off final victory over Sunderland, winning on penalties after an astonishing 4-4 draw, with Newton scoring Charlton's seventh penalty. Newton had earlier scored in the semi-final success over Ipswich Town to take them to Wembley.
The midfielder's first season in the top flight was interrupted by a knee ligament injury, but, although he recovered, the club's fortunes did not and they were swiftly relegated. He regained his spot in the side in the 1999-2000 season as the club won promotion again at the first attempt as champions. He found him often on the sidelines in the Premier League though, as Claus Jensen was signed and youngster Scott Parker emerged.
Frustrated by a lack of first-team opportunities in the 2000-01 season as the team remained in Premier League, he requested a transfer in February 2001 and was placed on the transfer list. Although he remained at the club for a further six months, he finally left The Valley in August 2001, joining First Division Wolverhampton Wanderers for £850,000, rising to £1 million if Wolves were promoted with him registered at the club. In total, he had made 285 appearances for Charlton, scoring 26 goals.
Newton made a bright start at Wolves, scoring on his debut against Portsmouth and producing a career-best seasonal goal tally of 8. Despite this, a late slump saw the club fall away from the top into the play-offs, where they lost to Norwich City. However, the following season saw the midfielder win a third promotion to the top flight as the club won the play-offs, beating Sheffield United 3-0 in the final.
He was a first choice for the West Midlands club in the Premier League, something that had evaded him in his Charlton days, but Wolves only managed one season at this level. He retained his place as the 2004–05 season began, but the arrival of Glenn Hoddle as manager saw his opportunities diminish and he left the club in March 2005. In total, he made 130 appearances for Wolves, scoring 12 times.
Newton moved to West Ham United in March 2005 for an initial fee of £10,000. His transfer fee later rose to £125,000 after the club gained promotion to the Premier League via the play-offs in May 2005. This marked Newton's fourth promotion from the division and he was rewarded with a new two-year contract in July 2005. He appeared regularly for West Ham in their return to the top flight, albeit mostly as a substitute, and he was not selected for their FA Cup Final squad to face Liverpool at the season's end. His only goal for West Ham came during this season, the winner in a victory over Manchester City on 15 April 2006.
In July 2006, Newton was suspended from West Ham due to drug misuse. He remained with the club, but only made five further appearances under his recently arrived former Charlton manager Alan Curbishley upon his return. Instead, he joined Leicester City on loan in March 2007 until the end of the season, a move made permanent in the close season. He made a total of 49 league and cup appearances for West Ham, scoring once.
Newton scored his first goal for Leicester against Birmingham on 17 April 2007. He was given the number 16 shirt, the third player to wear the number that season after Josh Low and Luigi Glombard. He signed a permanent one-year deal with the Championship club on 6 July 2007 after the appointment of Martin Allen. However his time at the club came to an end in January 2008, with Leicester now onto their third manager of the season in Ian Holloway, when his contract was terminated by mutual consent.
Newton spent a spell on trial at Yeovil Town in the spring of 2008 but was not offered a contract.
Charlton Athletic Play-offfinal winner 1998 Championship Division winner 1999-2005
West Ham UnitedFootball League Championship play-offs (1): 2005
Wolverhampton WanderersFootball League Championship play-offs 2003 Football League Championship play-off Final
In 2008 Newton was found guilty of nine counts of attempting to pervert the course of justice. Newton had attempted to bypass the speeding ticket procedure for fellow footballers, Teddy Sheringham and Bobby Zamora by "losing" the tickets which had been sent to them. He was given a 28-week sentence suspended for two years and ordered to do 180 hours unpaid community service. Newton was also ordered to pay £1,939 in costs and was disqualified from driving for one year. In July 2017, Newton was found guilty of burglary committed when he was drunk and high on cocaine. He was sentenced to 100 hours community service.