Simon is an exciting young Welsh central midfielder who has represented his country at Youth, B, U21 and Full International levels. He established himself in Peterborough's first team in 1998/99 and looked more than comfortable. Another to have risen through the London Road ranks after following Kit Carson to Peterborough from Norwich according to the October 1999 edition of Four Four Two. He had been travelling from Haverfordwest in Wales to Norwich every weekend in an attempt to make the grade but was one of 25 youngsters to follow Carson to London Road, Peterborough. He is a gritty performer who is not afraid to tackle and is also capable of changing the game.

Simon made his Welsh Under 21 debut in October 1998 matches against Denmark and Belarus. Outings against Italy and Denmark impressed Neville Southall who drafted him into the senior squad for the European Championship qualifier with Denmark in June 1999. A player with a big future who won both of the Peterborough's main 'Player of the Season' awards. Along with Matthew Etherington, he was included in last season's PFA Division Three team of the campaign. He attended trials at Old Trafford with Etherington in the summer of 1999 with a figure of £1.5million being mentioned for his services. The two played for United in a friendly against Boca Juniors but when United failed to involve them in a friendly with Omagh Town, the two youngsters returned to Peterborough to prepare for the new season.

The tabloid press reported two possible transfers for Simon during the weekend of 6th - 7th November 1999. Sunderland were supposed to have made a £1million bid for him whilst Aston Villa had proposed a joint £4million move for him and fellow Posh starlet Matthew Etherington. At the end of December 1999, Teletext reported that Spurs' bid of £1.2million for Davies and Etherington had been accepted by Peterboro' chairman Peter Boizot in that it was the only genuine offer received. Davies is said to be worth £700,000 and Etherington £500,000 as they step up into the Premiership.

Although neither player had broken into the Tottenham first team by early February 2000, Simon was called up to the full Welsh squad for the February friendly in Qatar. Davies and Etherington made their Tottenham debuts on 9th April 2000 against Liverpool. Simon has made 16 appearances for Spurs in the Premiership by the end of 2000/01 and made his debut for the Welsh national side against the Ukraine. By the end of 2001/02, he had added another 31 Premiership appearances for Spurs and added 4goals to the 2 scored the previous season. He won further Welsh caps against Armenia, Norway, Belarus, Argentina, Czech Republic and Germany.

Simon's impressive form for Wales and Tottenham saw him awarded the Welsh Footballer of the Year Award in October 2002. He celebrated this on 16th October by scoring Wales' first goal in their historic 2-1 victory over Italy (Craig Bellamy was the other scorer). This led to speculation in December 2002 that Liverpool manager Gerard Houllier would approach Spurs to swap England striker Emile Heskey for the young Welsh wizard. On 20th December, the Daily Express expanded this story to claim that Manchester United and Newcastle had also watched Davies, with the former alleged to be preparing a £9m bid. Spanish sides Barcelona and Athletico Madrid were also reported to have watched Davies play for Wales. This rumour continued over Christmas 2002 and into January 2003 with Man Utd said to be preparing a £8m bid. Speculation at the end of April 2003 suggested a £10million bid by Chelsea for Davies during the summer of 2003.

On 8th April 2005, the People speculated that Simon would join Everton in the summer of 2005 for £2.8m. This came true towards the end of May although the final figure was closer to £3.5m.

On 4th December 2006, the Daily Mirror reported that Everton were willing to offer Davies to Fulham in return for Louis Boa Morte. Davies did indeed join Fulham during the Janaury 2007 transfer window but for an undisclosed fee.

On 9th August 2010, Davies announced his retirement from international football. The 30-year-old Fulham player won 58 caps for Wales, scoring six goals. The Football Association of Wales said Davies had decided to concentrate on club football after struggling with a foot injury for the past 18 months. Davies had slipped down the Wales midfield pecking order due to emergence of Aaron Ramsey, David Edwards, Jack Collison, Joe Ledley and Andy King.

At the beginning of June 2013, Fulham announced that they would not be renewing Simon's contract when it expired at the end of the month. He had played 169 times for Fulham scoring 17 goals.

Davies returned to Peterborough in December 2018 as Professional Development Coach in the Academy, replacing Sam Darlow. Academy Manager Nick Sheppard was delighted with the appointment. “It’s great to have Simon on board, we all know what an incredible playing career he had, coming through the academy here and going on to play hundreds of games at the very top. Not only does he bring a wealth of playing experience but more recently, Simon has been working with our own under 13s and the U18s at Fulham. I am very much looking forward to working with Simon.”

His former colleague Matt Etherington had also returned to Peterborough as Professional Development Lead Phase Manager. Davies said “I must admit when I put the training wear on with the Posh badge, it was a proud moment. It has all happened so quickly. I spoke to Matt at the weekend after a departure within the set-up and it is a great opportunity for me and one I am really looking forward too. Myself and Matt get on very well and hopefully we can work well together and help the youngsters at the football club. “I, alongside Matt, started my career at Peterborough United within the Academy. I enjoyed my time here, I spent a long time at the club and it is great to be back. I can’t wait to get started,

When Etherington was appointed manager of Crawley Town on 27th November 2022, Davies joined him as Assistant Manager. They left the club on 30th December 2022 (32 days later) after winning one and losing two games. "It has become clear to all involved parties that this partnership is not the right fit to carry the club forward and achieve our goals" said director of football and interim chief executive Chris Galley. "As a result, we have mutually decided to move in a different direction."