The Parramatta Marist Old Boys provides all ex-students the opportunity to keep in touch and support the school community.
The Parramatta Marist Old Boys Union was founded in April 1926, after 325 old boys of the school gathered for a ‘smoko concert’.
The reunion was so successful that they resolved to form a committee for social purposes and support the progress of the School. Almost £100 was raised on that evening for the proposed science room in the school. The first Old Boys Union President was Mr Andrew Creagh.
The Union continues to this day and holds an Annual Reunion Dinner in October and all funds raised go towards the Br Coman Sykes Memorial Scholarship (awarded on an annual basis) and the archiving, preservation, framing and display of memorabilia within the school.
In 2016, the Old Boys Union established a tax deductible scholarship fund to greater fulfil its aims as an alumni organisation.
- Stan Wickham
Wallaby player and captain
- John Muggleton
Rugby League player for Parramatta, Balmain, NSW and Australia. Former defence coach of the Australian Rugby Union Team, the Wallabies, the ACT Brumbies and Melbourne Rebels.
- Mick Watson
- Mick Keelty APM
Former Commissioner of the Australian Federal Police.
- Geoff Brown
Australian Davis Cup tennis player and Wimbledon finalist in singles, doubles, and mixed doubles.
- John Devitt
Dual Olympic Gold Medallist swimmer.
- Paul Lynch
Sprint canoeist and Olympian.
- John Stephens
Professional Baseballer and Olympic Silver Medallist.
- Paul Hogan
Australian actor and comedian.
- Sir John Clancy KBE, CMG (1895–1970)
Australian judge and Chancellor of the University of New South Wales (1960–70).
- Denis Fitzgerald AM
Australian representative rugby league player and former CEO of Parramatta Eels.
- Chris Warren
Former professional rugby league player, television, radio, and sports commentator
(son of Ray Warren).
- Professor Chris O'Brien AO, (1952–2009)
Australian oncologist and surgeon.
- Jeff McMullen
Australian journalist, author and humanitarian
- Nathan Cayless
Australian Schoolboys, Parramatta Eels and NZ representative rugby league player.
- Jason Cayless
Former professional rugby league player and NZ representative rugby league player.
- Chad Robinson
Former professional rugby league player.
- Paul Gallen
Australian rugby league player player.
- Jamie Lyon
Rugby league player and former NSW and Australian representative.
- Daniel Irvine
Former NRL Rugby League player.
- Luke Ford
- Tony Ward
Actor and TV presenter.
- John Williams
Former National Rugby League (NRL) player.
- David Williams
Australian representative rugby league player.
- Kwabena Appiah-Kubi
A-League footballer representing Incheon United FC.
- Josan Nimes
Philippine Basketball Association (PBA) Shooting guard.
- Daniel Anderson
Former coach of the Parramatta Eels and St Helens and current NRL referees boss.
- Major General David Valentine Blake
Australian military officer commanding the unit who shot down and subsequently buried the Red Baron (WW1) and, the most senior officer present at Darwin in 1942 when the Japanese first bombed the city.
- Brian Tamberlin QC
Justice of the Federal Court of Australia (1994 - 2009). Prominent barrister, law commentator and alumnus of Harvard Law School.
- Professor Michael Gracey AO
World-renowned paediatrician, expert in Indigenous health and former head of the International Paediatric Association (IPA).
- Peter Arcadipane
Automotive Designer who designed the Mad Max Pursuit Special, CLS Mercedes and currently chief designer with Beijing Automotive Group (BAIC).
- Jack Ferguson
Politician and former Deputy Premier of NSW (1976-1984).
- Dan Mahoney
Politician and Member for Parramatta in the NSW state Parliament (1959-1976).
- Fred Cahill MBE
Politician and Member for Young in the NSW state Parliament (1941-1959).
- Pat Flaherty
Politician and Member for Granville in the NSW state Parliament (1962-1984).
- Andrew Ziolkowski
Politician and Member for Parramatta in the NSW state Parliament (1991-1994).
- George Thomas Ford
Politician, and Member of the NSW Legislative Council (1964–66).
- Edward Fleming
Chief Commissioner of the City of Sydney (1928).
- Cardinal Edward Clancy
Roman Catholic Archbishop of Sydney
- Bernie McGann (1937 – 2013)
Australian jazz alto saxophonist.
- Terry Wilkins
Musician and composer