Mike Phillips scored a try on his Welsh test debut against Romania in August 2003. His performances in the 2006 RBS Six Nations, as a regular replacement for the injured Dwayne Peel, confirmed his place as the number two scrum half for Wales.

Phillips was a notable success on the 2004 North American tour scoring against the USA in the 77-3 trouncing. Youth honours include winning the Triple Crown with the U21 team in 2002.

Phillips began his career at the Llanelli Scarlets and moved to the Cardiff Blues in the summer of 2005. During the 2005–2006 and 2006–2007 seasons Phillips made over fifty appearances for the region and scored eight tries.

After two seasons with the Blues, Phillips signed a three year contract with rival region the Ospreys, in a deal estimated to be worth £180,000 a year, to begin in the 2007–2008 season.

Featuring in the 2006 Invesco Perpetual Series against the Pacific Islands and as a replacement in the matches against Canada and New Zealand, Phillips also starred in the 2007 RBS Six Nations. He was a replacement in the opening match against Ireland, in the clash with France and in the victory over England.

Phillips was named in Gareth Jenkins's squad for the two-Test tour of Australia in May 2007. He appeared in both Test losses to the Wallabies and was later included in Wales's 41-man preliminary Rugby World Cup 2007 summer training squad. In August Phillips made a replacement appearance against England, was selected in the final cut of 30 for Wales's RWC squad and also played the second half of the Invesco Perpetual Summer Series against France at the Millennium Stadium.

Phillips made substitute appearances for Wales at the Rugby World Cup in the matches against Canada, Australia and Fiji, and started the game against Japan at the Millennium Stadium in which he scored a try.

Wales Caretaker Coach Nigel Davies selected Phillips on the bench for the Prince William Cup match against South Africa in November 2007, Phillips replaced Dwayne Peel in the second half of the match that the home side lost 34-12 to the reigning world champions.

In January 2008 Phillips was selected in Warren Gatland's RBS Six Nations squad. The scrum half played a part in all five matches of the Grand Slam winning tournament, scoring the match winning try in the opening encounter against England at Twickenham.

Phillips sustained a tear to the anterior cruciate ligament and a grade-two tear to the medial collateral ligament in March 2008 following the Ospreys' EDF Energy Cup semi-final victory over Saracens; the injury ruled him out of action for nine months and subsequently, Wales's summer tour to South Africa and the end of year fixtures against South Africa, Canada, New Zealand and Australia.

Phillips recovered from injury to play to be selected for the 2009 Lions tour of South Africa. He won selection for all three Tests (scoring a try in the first Test) to confirm his status as one of the best scrum half’s in world rugby.