FitzSimons first played club rugby with the Sydney University Football Club and then with the Manly RUFC in Sydney in the 1980s under the coaching of Alan Jones. Between 1985 and 1989 he played with CA Brive in France for four seasons as the club's first foreign player. He played seven test matches at lock for the Australian national rugby union team between 1989 and 1990, debuting against France in Strasbourg in November 1989, on the Wallabies 1989 tour of Europe. His final Test match was against the All Blacks in Christchurch. Former Wallabies winger David Campese criticised FitzSimons for starting a brawl in Australia's first Test against France in 1990. Campese labelled FitzSimons' actions "a disgrace to the good name of rugby" and asserted that "he was doing the game and its reputation enormous damage." Campese cautioned that if such fights "turn even one family away from the game, then they have been too costly". Former Wallabies backrowerWillie Ofahengaue said of FitzSimons: "He's a big character. Funny guy. Talkative. One thing I remember about rooming with him was he used to get his suitcase, tip it up and pour everything out on to the floor. When it was time to go home he would chuck everything back in any old way. Fitzy was a real roughie, but he is married now so he must have changed."
Journalist
FitzSimons has written for The Sydney Morning Herald since 1988, and has been a sports columnist for that publication since 1987. He regularly appears on the Australian Foxtel programme, The Back Page, formerly hosted by rugby league journalist Mike Gibson and now Tony Squires. For the Saturday edition of The Sydney Morning Herald, FitzSimons writes a column titled "The Fitz Files" which looks at all the happenings over the past seven days in sport. He writes a more general version of "The Fitz Files" in The Sun-Herald on Sundays, focusing on community activities and events in Sydney. Andrew Denton has called him "Australia's finest sports journalist". On the 25 September 2001 he wrote a thought provoking opinion editorial piece titled Memo world: try saying sorry to avoid a sorry end.
Radio
In January 2006 FitzSimons began co-hosting a breakfast radio program with Mike Carlton on Sydney radio station2UE. He was brought onto the 2UE breakfast show in an attempt to boost the program's dwindling ratings. However, the Mike and Fitz Breakfast Show still trailed a long way behind the number one program on 2GB, hosted by FitzSimons' former coach Alan Jones. After two years, FitzSimons quit to become a stay-at-home dad and focus on his writing.
FitzSimons is married to Australian journalist and TV presenter Lisa Wilkinson. They have three children and live in Sydney. FitzSimons has identified himself as an atheist and a republican.
Honours
On 13 June 2011 FitzSimons was named a Member of the Order of Australia for service to literature as a biographer, sports journalist and commentator, and to the community through contributions to conservation, disability care, social welfare and sporting organisations.