Born in Mansfield, he came through the Derby County youth team ranks before turning professional in 1997 before joining Sheffield United in March 1999.
The following month, he won a second start, helping the club beat Tottenham Hotspur 2–1 at White Hart Lane in the next round of the competition.
Kozluk gained two caps for the England under 21s in summer 1998,[4] where he played in the Toulon Tournament alongside Frank Lampard and Emile Heskey.
The following season, retaining his first-team place under new manager Neil Warnock, Kozluk made 43 appearances, however, at the start of the 2000–01 campaign he joined First Division rivals Huddersfield Town on a three-month loan deal.
Having made fourteen appearances for the Terriers, he returned to Sheffield United, where he managed to win back his first-team spot and played regularly until the end of the season.
Back in the first team in 2002–03, he played 35 league games scoring his first-ever senior goal with a 30 yards (27 m) strike against Grimsby Town on 4 March.
Kozluk made a total of 47 appearances in 2003–04, as the Blades recorded an eighth-place finish, during which he also scored the second goal of his career when he knocked one past Stoke City's Ed de Goey at the Britannia Stadium on 1 November.
A further 20 first-team appearances followed in the top flight the following season, but the club failed to consolidate their top-flight status and were relegated in a final day showdown with Wigan Athletic.
[2] Helping the club to avoid relegation by the end of the 2007–08 season,[2] he also played a role in the club reaching the FA Cup FA Cup semi-finals,[2] helping the Tykes to overcome Liverpool at Anfield,[11] and Chelsea at Oakwell,[12] before ultimately losing 1–0 to Cardiff City in the semi-finals at Wembley.
[14] Signed as a squad player he made little impact on the Blades first-team until he was given a run of games by new manager Micky Adams in January.
[29] Manager Kevin Wilson said: "Although Rob has not been playing this season, he has been training regularly with Chesterfield and, in my opinion, he is still more than capable of doing a job for a Football League club.