He married in January 1693 at Corrie Arklet farm near Inversnaid Mary Helen MacGregor of Comar (1671-1745), who was born at Leny Farm, Strathyre. They had four sons: James (known as Mor or Big), Ranald, Coll, and Robert (known as Robin Oig or Young Rob). A cousin, Duncan, they later adopted.
Rob Roy became a well-known and respected cattleman — this was a time when cattle rustling and selling protection against theft was a commonplace means of earning a living. Rob Roy borrowed a large sum to increase his own cattle herd, but due to the disappearance of his chief herder, who was entrusted with the money to bring the cattle back, Rob Roy lost his money and cattle, and defaulted on his loan. As a result, he was branded an outlaw, and his wife and family were evicted from their house at Inversnaid, which was then burned down.