His person and followers were in the Highland army, but his heart seems to have been with the Duke of Argyle's.

Yet the insurgents were constrained to trust to him as their only guide, when they marched from Perth towards Dunblane, with the view of crossing the Forth at what are called the Fords of Frew, and when they themselves said he could not be relied upon.
This movement to the westward, on the part of the insurgents, brought on the battle of Sheriffmuir--indecisive, indeed, in its immediate results, but of which the Duke of Argyle reaped the whole advantage.

In this action, it will be recollected that the right wing of the Highlanders broke and cut to pieces Argyle's left wing, while the clans on the left of Mar's army, though consisting of Stewarts, Mackenzies, and Camerons, were completely routed.

During this medley of flight and pursuit, Rob Roy retained his station on a hill in the centre of the Highland position; and though it is said his attack might have decided the day, he could not be prevailed upon to charge.

This was the more unfortunate for the insurgents, as the leading of a party of the Macphersons had been committed to MacGregor.