After the American Revolution, John Silverthorn moved to Upper Canada from New Jersey as a United Empire Loyalist. He settled first in the Niagara Peninsula in 1786, then moved to Cherry Hill Farm located north of Dundas on Cawthra Road in Toronto Township (Mississauga), but later, as his name was added to the loyalist list in 1808, was given a land grant from a grateful government in Etobicoke Township. This property, known as "The Old Mill Place", or "Mill Farm", has belonged to the Silverthorn family until the 1960's when the Markland Woods development was begun. On this property, John Silverthorn built a mill in about 1810 down at the end of what is now Mill Road, where Mill Road meets the Etobicoke Creek. There used to be a bridge across the creek on Mill Road, but this was washed out during Hurricane Hazel and has never been replaced. The mill was located on the creek close to Mill Road.

Mr. Silverthorn carried on a milling business the rest of his life at the "Old Mill Place". Mill Road was built by him as a "given" road from Burnhamthorpe Road to Dundas Street for those doing business at the mill.

When the area was developed as Markland Woods in the early 1960's the Board of Education for the Township of Etobicoke decided to build a secondary school for the area. The school was to be named Silverthorn Collegiate Institute after the family who had first settled the area. The cornerstone was laid in 1963 and the school opened in September 1964 with a teaching staff of 24 and a support staff of 7. Murray Young was the first principal. The school had only grades 9, 10, and 11 in 1964. In the next two years it added grades 12 and 13.

In 1968-69 a massive new addition was added to the school effectively doubling its size. An auditorium, the present library, science labs and technical and business areas were added at that time. A second gym was added later.

Today the school is comprised of 1,250 students and 81 staff. In 1999, SCI celebrated its 35th year of operation and it remains a central part of the community and the focus of daily life for many of its students.