Donald Sr did not have an easy war. By October 1915 he was fighting in France and Belgium. In March 1916 he contracted paratyphoid. At the Somme in July 1916, he was buried by a shell explosion which left him shell shocked and he returned to England that October where he was reassigned to non-combatant work. When he was transferred to Hastings in March 1917, he met Nellie Charlotte Hilder who was eight years his junior. They married on July 17, 1917. Donald Sr ultimately completed the war as a regimental sergeant-major at the 1st Canadian Command Depot.

Nellie gave birth to Donald Jack Smith on April 19, 1918 in Hastings. The couple may have had a daughter in 1919 named Nellie, who died in infancy. When Donald Sr was shipped back to Canada that year, he had to leave his wife and his young son behind, making arrangements for the two to join him in Canada.

Once he returned to Canada and was discharged, Donald Sr's shell shock symptoms still plagued him. It was decided that he shouldn't return to his farm. His brother who farmed with him had died in the war. It is unknown what happened to the children in Mrs. Foster's care in Charlton.

Donald Sr remained in Toronto and Nellie and Donald joined him in Lambton Mills in 1920, living on Government Road. A daughter, Mabel Doreen, was born that year in Toronto and another son, Ronald George arrived in 1923. By 1924 they were living at 20 Marquis Avenue in Lambton Mills. At the time, the street was known as Oakview Avenue. Donald Sr, a member of Lambton Mills' volunteer fire department, was injured on March 11, 1924 when two gasoline tanks blew up on the north side of Dundas Street, west of Scarlett Road. The blast rocketed one of the tanks 350 metres south into a market garden in Lambton's valley. Donald Sr was admitted to Toronto Western Hospital, having burnt his head and hands. Eight other men were burned, one seriously.

Donald Sr was a messenger for the Sterling Bank branch at Chestnut Street and Dundas and he was also a notary public. He became a pillar of the Lambton Mills community in the ratepayers association and as chairman of the Lambton Mills and Kingsway school board in the 1930s.

Donald Jr began his school career in the mid-1920s at Lambton Mills school, a stone's throw from his house. He probably graduated in the school's Class of 1932 and continued to high school at Etobicoke. He grew to be a tall boy – 6'4”, much taller than his father's 5'7'. He played baseball (infield), basketball (centre) and rugby (middle) in high school and although he had a splendid voice, he wasn't musical. He did enjoy the arts and was a member of a dramatic society. His true passion was the outdoors, whether fishing or hunting pheasant, rabbits and deer. As an adult, he collected firearms. Donald would also swing a club around a golf course occasionally.

In 1936, his parents moved the family across the road into 11 Oakview Avenue. The street wasn't renamed Marquis until 1939, to differentiate it from Oakview Avenue in the High Park neighbourhood.

In 1935, Donald left Etobicoke in Grade 11 with his junior matriculation. He became a statistical clerk at Canadian General Electric downtown at King and Simcoe Streets. For two years he continued school at night in Runnymede Collegiate's Business Administration program, where he learned typing and shorthand. When war was declared in September 1939, Donald joined the reserves of the Queen's York Rangers. Like many young men at the time, his sense of duty was also influenced by his father's bravery in the First World War.

He resigned his 2nd Lieutenant commission to join the Irish Regiment of Canada as a private, enlisting on June 10, 1940. He stated that after the war that he did not wish to return to his $28 per week job at General Electric, hoping instead to become a salesman or to be involved with a personnel department or with a youth organization.

Donald showed good leadership skills and was quickly promoted to sergeant the next month, ultimately becoming an acting platoon commander. The regiment remained in Toronto for local protection duties until January 1941, when it moved to Camp Borden near Barrie. A course that Donald took in November 1941 was held at Long Branch in today's South Etobicoke. Soon after this Donald moved to Nova Scotia to join his regiment which had been serving in Debert since the middle of August. Not long after Donald arrived, the unit moved again, at the beginning of December 1941, to Mulgrave, across from Cape Breton Island, where the regiment provided coastal protection of the Strait of Canso. Donald would have enjoyed being in Mulgrave as fishing parties were sent out and caught trout for the mess. Forest fire fighting was also undertaken. The regiment had a baseball team that played navy teams from ships anchored in Mulgrave and it was so rural that cows had to be shooed off of the company firing range before shooting practice could begin.

At the beginning of August 1942, the unit shifted to Shelburne, on Nova Scotia's south coast, continuing its coastal defence assignment. Several days beforehand, Donald was promoted to acting company sergeant-major. The Irish Regiment knew it was to ship out soon when it returned to Debert in September and embarkation leaves were granted. Donald returned to Toronto for a week in mid-September and the unit sailed from Halifax on October 28, on the RMS Queen Elizabeth, a passenger ship which had been stripped of its luxuries so that it could carry troops. The unit disembarked on the Clyde River in Scotland, arriving at Aldershot, the huge army training camp 60 kilometres southwest of London, on November 6.

Donald was promoted to full company sergeant-major and given leave the first week of December. Whether he visited London or his English relatives is unknown. On Christmas Eve, his commanding officer recommended Donald for Officers' Training Corps (OTC), writing “Is ambitious and anxious to get ahead.” Donald had another seven days' leave in mid-January 1943 and the transfer to OTC was granted by late winter. Donald was back in Toronto on March 29. He was given leave from April 3 to 16 and began his course in Brockville, Ontario on April 23. His brother Ronald was in training with the RCAF. Donald had returned to Toronto at the same time as Jack Stewart, whose nickname was “Lucky.” He had enlisted in the Royal Hamilton Infantry in September 1939 and was a veteran of the disastrous Dieppe raid in August 1942. Presumably the two men became friends.

The Brockville course was 90 days long, and, along with classroom courses in subjects like map reading and public speaking, there was training on an assault course, driving lessons over rough terrain in trucks and on motorcycles, hand-to-hand combat and cliff climbing, among other skills.

Donald had a girlfriend, Mabel Norma Risebrook who was 20 years old in 1943. Her father was a policeman and she lived on Brookside Avenue, near Jane and Dundas Streets. Presumably she met Donald when he was studying at Runnymede. In June, Donald asked Mabel to marry him and was granted permission to do so by the army.