McMackin directed over seventy-three films between 1912 and 1916 directing films such as When Empty Hearts Are Filled and The Altar of Ambition in 1915 working with actors such as Harry von Meter, Louise Lester, Vivian Rich and David Lythgoe.
His career reached its height in 1916 where in that year alone he directed thirty short films.
With the death of MacMackin's father in 1900, the family faced extreme financial hardship.
In May 1913, a portion of the Essanay film company moved from Chicago to Ithaca, New York – among them was MacMackin who was now working as an assistant director.
By 1917, MacMackin had directed seventy-four pictures and was working at Edward Small Inc. in New York City where he lived with his family.
He worked as a writer on his final film, the 1920 comedy The Rookie's Return, starring Douglas MacLean.
MacMackin (along with his second wife) later converted to The Church of Jesus Christ of Latter-day Saints.