In 1843, a grammar school with 40 paying students was opened in the Sandy Hill area of Ottawa in a house at the corner of Waller Street and Daly Avenue.

In 1908, Ottawa architect Edgar Lewis Horwood added a west wing with laboratories, an auditorium, and the main tower.

[5] The auditorium balcony is suspended by iron rods which lead to huge beams above the ceiling.

The Gym Building also includes the school's computer science labs, music rooms, and its communication technology classrooms.

[citation needed] In the 1970s, the Ottawa Board of Education decided to close the school and sell its downtown real estate.

To meet fire code requirements, the existing west staircase was moved from the stair tower westward to just beside the entrance to the auditorium.

In March 2003, parts of the first and second floors and the basement of the North building were damaged by a water main break that closed the school for a week, coincidentally before the previously scheduled March Break, thus giving the students two weeks off school.

A brass plaque and print were erected by students and alumni in 1986 to Sergeant Edward James Gibson Holland, VC of The Royal Canadian Dragoons, a graduate of Ottawa Collegiate who was awarded the Victoria Cross for bravery in action during the Boer War at Leliefontein, Komati River, South Africa on November 7, 1900.

[13] A memorial framed poster erected by the school is dedicated to the sixteen Canadians awarded the Victoria Cross for conspicuous bravery during the Second World War.

[18] The school serves the neighbourhoods of Sandy Hill, New Edinburgh, Centretown, Rockcliffe Park, and part of Old Ottawa East.

[20] Lisgar was ranked number one for public schools in Ottawa and 4th in the province by the Fraser Institute in 2008.

[22] Students have frequently placed in the top ten amongst Canadian Mathematical Olympiad winners.

Sports at Lisgar include rugby, soccer, hockey, basketball, rowing, cross country.

These include the co-presidents, Student Senator, Secretary, Treasurer, and 5-6 elected grade representatives, alongside 6 committees and several individual roles consisting of the remaining positions.

Student Council is responsible for running a variety of school-wide events and activities throughout the course of the school year, including the Annual Pancake Breakfast, the week-long 'Battle of the Grades', and the end-of-year 'Oscars'.

The school's Reach for the Top team won the Canadian national finals in the 2008, 2015 and 2017 seasons.