Electric Scotland gives the following as the origin of the surname (McKinley) MacKinlay.
The name MacKinlay comes from the Gaelic form of Findlayson meaning "son of Findlay" or MacFhionnlaigh. The name is not common despite being distinctively Scottish but is also to be found as MacGinlay in county Antrim in Northern Ireland and later in North America. The clan county of the MacKinlays was the Lennox district, north of Callender and they descended originally from Findlay a son of Buchanan of Drumikill. According to Buchanan of Auchmar some of the MacKinlays were also connected to the MacFarlanes. It is possible that some of the MacKinlays came from the same stock of the MacKinlays as the Farquharsons of Braemar, as the clan ancestor was Fionnlagh Mor and as a clan are called in Gaelic "Clann Fhionnlaigh" (This cannot be substantiated because there were no MacKinlays in Braemar or its vicinity). There are several spellings of the name that appeared in the 17th century especially in Glenlyon and Balquidder. Although it has been suggested it is not very probable that the name MacKinlay embraces some of the MacLeays "sons of Donleavy" with its variants the same as the MacKinlays; Findlay, Finlayson, Donleavy and Macinlay. The MacKinlays have been connected to the Farquharsons, the Buchanans and sometimes the Stewarts of Appin. From the MacKinlays descends William McKinley (1843-1901), the 25th President of the United States of America who also gave his name to Mount McKinley in Alaska.