Returning to Scotland, Rennie continued to play for Scottish Junior Football Association teams Dunipace and Bo'ness whilst working in sales and marketing for Blackbaud.

With the money earned from his sales and marketing work, Rennie was able to personally pay for his SFA coaching course costs.

Rennie won the coach of the year award in a season where the team conceded a USL all-time record low of 13 goals.

Some of the top young players that Rennie had recruited and coached in 2007 were transferred to professional teams at a higher level before the 2008 season began.

With the Carolina RailHawks he inherited a team that had only made the playoffs once in the history of the Franchise and had never finished above eighth place in a regular season campaign.

He then completely changed the team culture and environment, introducing psychological, tactical and physical concepts that had never been implemented at this level before.

[9] In the 2013 season Rennie continued to revamp the playing squad, reduced the age of the team and introduced young players like Kekuta Manneh to the Whitecaps lineup as well as Premier League experience in former England U21 international Nigel Reo-Coker.

[13] Bob Lenarduzzi, president of the Whitecaps cited inconsistent performances and tactics as well as a failure to win the Canadian Championship as reasons for his dismissal.

[14] On 17 July 2014, he was appointed as manager of newly formed South Korean side Seoul E-Land FC who were scheduled to join K League Challenge in the 2015 season.

[16] On 16 January 2018, American club Indy Eleven announced Rennie as head coach ahead of their inaugural United Soccer League (USL) season.

[18] On 10 December 2021, Rennie would return to his native Scotland, being announced as the new manager for Scottish League One side Falkirk.