He also played for Hamilton Academical, Millwall, Raith Rovers, Hibernian, Greenock Morton, St Johnstone, Bristol City and Aberdeen during his career.

After signing 17 players in three months, he was unable to register a league win in the first nine games of the season and he departed with the club in 23rd position in the table.

Millwall paid £380,000 to gain his services in July 1996; this remained Hamilton's biggest sale until James McCarthy moved to Wigan Athletic for an initial £1.2 million in 2009.

[5] Hartley returned to Scotland a year later, joining Raith Rovers for £150,000, before moving to Hibernian in 1998, whom he helped win the First Division in 1998–99.

This change of position from his previous role as a right winger, coincided with a significant upturn in his performances, but it was not enough to prevent St Johnstone's relegation to the First Division.

Hartley's personal success continued as he was nominated for the SPFA First Division player of the year award in 2003,[9] but his team failed to gain promotion back to the SPL in 2003.

[citation needed] Hartley scored three goals against archrivals Hibernian in the Scottish Cup semi-final in 2006, his first hat-trick as a professional footballer.

[17] On 26 January, however, Hearts and Ivanauskas admitted that they had now come to the decision to sell their prize assets; both Hartley and Craig Gordon were both dropped for that weekend's match against Rangers.

[24] Hartley played his first match for the club in a friendly against Dutch team Ajax before scoring on his competitive debut in a 2–2 draw away to Preston North End.

[30] It was the first time a player had scored a hat-trick of penalties in a Scottish top division match since Donald Ford, for Hearts against Morton, in September 1973.

[34] Hartley's first experiences of international football took place right at the beginning of his professional career when he was selected by manager Craig Brown as a training squad player for Euro 96.

[47] Hartley joined newly promoted Scottish League Two club Cove Rangers in July 2019, succeeding John Sheran as manager.

[52] On 30 August, he won a competitive game for the first time at the eighth attempt, a 2–0 home win over Harrogate Town in the EFL Trophy group stage.

[53] On 18 September 2022, following a 2–0 defeat to Sutton United, Hartley was sacked by Hartlepool following a poor start to the season, with the club being in 23rd place at the time of his departure.

[55] On 5 January 2023, Hartley returned to Cove Rangers, succeeding Jim McIntyre as manager and signing a contract until June 2026 with the club.