[1] Now the Gillespie Heritage House, 'Earlston' was previously owned by Sir Charles Mackellar, was designed by architects Spain & Cosh, and built in 1908 for W. Moses Esq., Warrawee.

[11] Under the founding headmaster Neil MacNeil, a Rhodes Scholar, Knox grew rapidly and survived the Great Depression.

The school's fifth headmaster, Ian Paterson, initiated further developments including a substantial building program.

The KG1 Project The Prep School's K-2 Centre, opened in 2004, provided new classroom, library, art and music facilities for Years K-2 students.

The Great Hall and Aquatic Centre project,[17] was finished in August 2011 and won design awards in 2012.

In 2016, Headmaster John Weeks announced that Knox Grammar School would commence construction of a new Performing Arts Centre and Junior Secondary Academy in February 2017.

The Senior School's construction for a new locker area for Years 7-10 as well as a basketball court, which is in the process of refurbishing the KG1 Building for the new Middle Academy, was completed in early 2020.

The Unit participates in combined Annual Field Exercise (AFX) at the end of Term 1, and holds its own Junior, Senior, and CUOs Promotions Courses during August each year.

Students may represent Knox in a variety of inter-school sporting fixtures played each Saturday throughout the term.

[9] The Intra-School sporting programs includes House carnivals, Standards and Inter-School competitions open to all boys.

At the Corby Highland Games, The Knox Pipes and Drums won first place for every event that they entered in.

[25] The school attracted widespread media coverage in 2009, when criminal charges were laid against five former teachers for alleged sex offences between 1976 and 1990.

The commission will examine the "systems, policies and procedures" involving the school's response to the complaints since 1970 and the experiences of former students sexually abused by teaching staff.

The commission heard that in fact Paterson had never reported any student's allegation of sexual abuse to police during his thirty years in charge of the school.

[31][32] Paterson also stated that he had allowed several teachers accused of sexual abuse to resign and subsequently gave them positive references.

Weeks told the media that the allegations had not been detailed or specific, and he had received advice that "it would have been difficult on industrial grounds" to have dismissed the teacher.

The Sydney Morning Herald stated that its conclusions were that "Paterson deliberately covered up allegations about child sexual abuse because he placed the reputation of the school ahead of student welfare".

[37] The Royal Commission also found that principal Ian Paterson had a "dismissive" attitude towards complaints of sexual abuse, "deliberately withheld information" from a police officer investigating allegations made against Knox Grammar staff and did not notify the school's council or affected parents of complaints.

[38] Knox Grammar School has not signed up to compensate its sexual abuse survivors through the National Redress Scheme, though it indicated it plans to do so.

[39] On 5 August 2019, Nicholas Warby, the school's 30-year-old "Director of Aquatic Sports", was arrested for possession of "child abuse material" on his mobile phone.

[43] On 30 August 2024, William Gulson, an English teacher at the school and a former Year 7 Mentor, was arrested and charged with one count of procuring or grooming a child under 16 for unlawful sexual activity.

William Gulson has been granted conditional bail under the circumstances that he is not to enter the premises of Knox Grammar or come in contact with anybody under the age of 18.