During the First World War, Ernest John Hart enlisted in the 116th battalion and went overseas with that unit. He served in France and returned home in 1919. After his return home from overseas he was married to Miss Gertude Lymer of Maple Grove (now a part of Courtice, Ontario), and the couple had three sons and a daughter.
Ernest Hart was born in Bowmanville, Ontario. He lived all of his life, except for the period he spent overseas during the war, in the Oshawa and Harmony district, and was associated with his father and brothers in the Oshawa Dairy Company. Of mechanical turn of mind, he was in charge of the plant of the company and built it up to a high state of efficiency.
On April 23, 1933, at the age of 36, Ernest Hart died of pneumonia at the Oshawa General Hospital. He had been ill only a few days and only three days before his death he seemed to be on his was to a good recovery from his illness. He relapsed two days later, and came to an end peacefully the next day.