Gardiner was a prolific goalscorer for City, scoring a remarkable 49 goals in 71 games in all competitions. He started his career in reserve football in Scotland, however, and after failing to get much of a chance with Rangers he signed for City for £4,000 under manager David Halliday.

He wasted no time in finding the back of the net, scoring a goal a game in his first season with the Foxes, netting 34 goals in the Second Division. However, he lost his starting role in City's promotion campaign in 1957, being reduced to reserve team football. But he still scored 18 goals for that reserve side, enough to see him called back into action when needed to help City in Division One.

Gardiner moved on in 1958, with Reading signing him, although his stay there was hindered by two serious injuries. He finished his career with Sudbury Town and went on to be a telephone exchange operator for British Telecom.