Construction at the mental health facility continued until 1926 and by 1927 the capacity at Ontario Hospital, Whitby reached 1542 beds. In the years following the First and Second World Wars, the hospital serviced an area that at one time encompassed seven counties. By 1994, the hospital's 75th anniversary, approximately 2.2 million people received treatment at the hospital.

A number of physical changes were made to the facilities throughout the 20th century; buildings were constructed and demolished. In 1969, the farm was closed and its animals were auctioned-off. The hospital was expanded in order to provide treatment to children and a forensic unit was added to care for patients involved with criminal offenses.

Advancements in the types of treatment provided at the hospital also occurred during the period from 1919-1996. In the 1970s, the hospital administration redefined the function of the hospital by emphasizing the importance of prevention in mental health care. The focus of the hospital also shifted from a long-term treatment centre to an out-patient health facility that sought to reintegrate patients into their communities. It was during this time in 1968 that the hospital was renamed Whitby Psychiatric Hospital.