This line were known as the Lagalgrave MacMillans and allegedly did not have full appreciation of their standing as clan chiefs, although they served their country well. Duncan's brother was William MacMillan who served as a captain of the Marines under Admiral Nelson on his flagship HMS Victory.
The Lagalgrave MacMillans were recognized by this title; however, they reportedly did not fully understand their role as clan chiefs despite serving their country well. William MacMillan, Duncan's brother, served as a Marine captain aboard Admiral Nelson’s ship, the HMS Victory.