Hartley began his professional career in 1994 at Hamilton Academical and played two seasons with the Accies until Millwall piloted him for 400,000 pounds in the English league, where he played one year and ran aground also for the Scottish U -21 national team. In 1997 he returned to a release of 150,000 pounds to Scotland and played for Raith Rovers, before a season it Hibernian joined and rose with Hibs in the First Division in the Scottish Premier League.
1999/ 00 he was briefly loaned to Greenock Morton, but then joined St. Johnstone, but the descent at the end of the season.
During his time with the Saints worked him Billy Stark from winger to central midfielder for. This change was soon fruits and Hartley's performances were always better. In 2003 he returned to the House of Lords, when he signed a free transfer at Heart of Midlothian. In the Hearts Hartley continued to improve and helped them in 2003/ 04 to reach the third place and consequently the UEFA Cup.
In January 2005, Celtic offered £ 300,000 for Hartley, but what the Hearts rejected. He was then he signed a contract with enhanced remuneration.
In March 2005, Paul Hartley played his first international match against Italy for the Scottish national team. He received further appeals and was able to record his first goal in the National Dress in a 3-0 victory over Slovenia in October 2005.
One of Hartley's probably best performances was his three-goal game in the semifinals of the Scottish FA Cup against their arch-rivals of Hearts, Hibernian. This was his first hat trick as a professional. In the final, he was expelled after a weak performance of the field, the team from Edinburgh was against the then third division Gretna nevertheless victorious and won on penalties.
In January 2007, the Rangers from Glasgow and Aston Villa tried to Hartley's services. Hearts coach Valdas Ivanauskas rejected both offers. On January 26, it was confirmed that Paul Hartley is available for purchase and would not be in the squad for the next game. Hartley was finally sold on 31 January for 1.1 million pounds of Celtic. Hartley was able to win with Celtic twice the championship. With the end of the 2008/09 season, his contract was not renewed. Currently he is under contract at Bristol City.