Andersen was born to parents Mary and Danny in Melbourne where he started playing basketball at Frankston East Primary School.

His mother coached him and his brother, Stuart, at Frankston East and because there was only one team at the school, Andersen always played a year above his age.

[1] At the age of 11, Andersen joined his local basketball association, the Frankston Blues, and in his first year at the club, the under 12 team he played in, won the championship.

In January of the 2005–06 season, Andersen dislocated his ankle and fractured his fibula, forcing him to return to Australia for surgery, specialist treatment and rehabilitation, before heading back to Russia to continue rehab and to support his team for their 2006 EuroLeague Final Four campaign.

On 14 July 2009, the Atlanta Hawks traded Andersen's rights to the Houston Rockets in exchange for cash and future draft considerations.

[6] Although being drafted in 2002, Andersen did not play in an NBA game until October 27, 2009, where he recorded 11 points and 5 rebounds in his debut against the Portland Trail Blazers.

[7] On 28 July 2010, Andersen was traded, along with cash considerations, to the Toronto Raptors in exchange for a 2015 protected second-round pick.

[8] On 20 November 2010, he was traded, along with Jarrett Jack and Marcus Banks, to the New Orleans Hornets in exchange for Jerryd Bayless and Peja Stojaković.

[10] Andersen's final NBA game was on April 1, 2011, in a 81–93 loss to the Memphis Grizzlies where he recorded 2 rebounds and 1 assist.

[12] In August 2012, Andersen and Montepaschi agreed on a €800,000 buyout, and he subsequently signed a two-year deal with Fenerbahçe of the Turkish Super League (BSL).

[25] On 4 June 2018, Andersen signed with the Illawarra Hawks for the 2018–19 NBL season, returning to the franchise for a second stint, a full twenty years after his first.

[30] The following month, he won his second NBL championship after United defeated the Perth Wildcats 3–0 in the grand final series.

[1] In the 2003 off-season, Andersen was selected to play for the Australian Boomers in the Olympic Qualifying FIBA Oceania Championship against New Zealand.

[33] As a member of the Boomers' 2014 FIBA World Cup squad, Andersen averaged 6.3 points, 5.0 rebounds, and 1.2 assists in six games.