FitzSimons grew up in Peats Ridge, New South Wales, in the rural outskirts of Sydney. He was the seventh and last child of Beatrice Helen Booth (1920-1994) OAM and Peter McCloy FitzSimons (1916-1992), a citrus fruit farmer who had seen active service in WWII as an AIF artilleryman. He attended Peats Ridge Primary School, and Knox Grammar School and earned an Arts degree at the University of Sydney.

Rugby

FitzSimons' club rugby was played with the Manly RUFC in Sydney in the 1980s under the coaching of Alan Jones. Between 1984 and 1989 he played with CA Brive in France for four seasons, becoming the club's first ever foreign player. He played seven Tests at lock for Australia between 1989 and 1990, debuting against France in Strasbourg in November 1989, on the Wallabies 1989 tour of Europe. Five of his career international appearances were made against France. He is one of only two Wallabies ever sent from the field against the All Blacks, being sent off in a 1990 Bledisloe Cup match.

Journalist

FitzSimons has written for The Sydney Morning Herald since 1989 and is currently a sports columnist for that publication. He also regularly appears on the Australian Foxtel programme, The Back Page, hosted by rugby league journalist Mike Gibson. 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" on page two of the Sun-Herald on Sundays, focusing on community activities and events in Sydney.

Radio

In January 2006 he began co-hosting a breakfast radio program with Mike Carlton on Sydney radio station 2UE. He was brought onto the 2UE breakfast show in an attempt to boost the program's dwindling ratings. Mike Carlton was vocal in his opposition to having an on-air partner, but the move paid dividends with an immediate audience increase. 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 on Breakfast with Mike and Fitz, FitzSimons hung up the headphones to become a stay-at-home Dad and focus on his writing.

Literature

FitzSimons' published works include:

• Kokoda, which recounts the numerous battles between Australian and Japanese Troops on the Kokoda Track during World War II.
• Tobruk, which tells the story of The Rats of Tobruk as they fought during World War II against Italian troops, then later the Afrika Korps as they were led by then-General Erwin Rommel.
• Charles Kingsford Smith and those Magnificent Men - a biography of aviator Sir Charles Kingsford Smith.
• A Simpler Time a childhood memoir which relates his family history and honours the memory of his father and mother.

He has also written the biographies of former Australian Labor Party leader Kim Beazley, Wallaby captains, Nick Farr-Jones and John Eales, WWII resistance fighter Nancy Wake and sportsmen Steve Waugh and Les Darcy. In spite of some critical reviews by established historians, most of his books have sold well.

Boards

He is or has been involved with several organisations as a Patron or board member, including:

• Patron of AFS Intercultural Programs, a student exchange organisation; he was an AFS student to the United States in the 1970s.
• Patron of the Newport SLSC which his grandfather helped establish in 1913.
• On the Senate of Sydney University.
• On the Board of the Sydney Writers Festival since 2003.
• President of the Northern Suburbs Basketball Association with 4,500 members, the largest basketball association in Australia, outside of Victoria.
• Patron of the Benjamin Andrew Footpath Library, devoted to getting second-hand books to homeless people.

Fitzsimons is married to Nine Network Today Show host Lisa Wilkinson. They have three children Jake, Louis and Billi.

FitzSimons is an outspoken Australian republican and supports changes to the Australian flag.