MacWilliam was born in Edinburgh, Scotland, and immigrated with his family to Cleveland, Ohio, when he was 18.

[2] During World War I, serving with the 313th Machine Gun Battalion, 80th Division[6] at the Battle of Meuse-Argonne, MacWilliam was shot in both legs while rescuing a trapped and wounded soldier.

[2] He was also the victim of a mustard gas attack, and suffered continuing health problems because of it.

[5] His doctor mentioned to him that he was investing in land in Florida, and MacWilliam decided to join him.

[5] MacWilliam established the Indian River Mosquito Control Board, and introduced height restrictions on buildings.