[4] Giddey grew up in Yarraville and attended St Kevin's College in Melbourne from Years 7 to 10, before emerging as one of Australia's top basketball prospects with the NBA Global Academy, a training center at the Australian Institute of Sport (AIS) in Canberra.

[7] At the Australian Under-18 Championships in April 2019, Giddey averaged 20 points, 8.3 rebounds and six assists per game, leading VIC Metro to the title.

[3] In January 2020, Giddey helped the NBA Global Academy win the Torneo Junior Ciutat de L'Hospitalet in Barcelona, where he earned most valuable player (MVP) honors.

[20] That same day, he suffered an ankle injury five minutes into his NBA Summer League debut and was ruled out for the rest of the competition as a precautionary measure.

[27][28] Giddey was named the NBA Western Conference Rookie of the Month for games played in January and again in February, winning the award four consecutive times.

[33] On 15 January, Giddey scored a season-high 28 points along with nine rebounds and assists, leading Oklahoma City to a 112–102 win over the Brooklyn Nets.

[34] On 12 April, during the NBA play-in tournament, Giddey recorded 31 points, 10 assists and nine rebounds in a 123–118 win over the New Orleans Pelicans, advancing the Thunder to the eighth-seed game.

[42] Giddey was one of the final cuts from the Australian basketball team roster for the 2020 Tokyo Olympics and was instead selected as one of three emergency players.

[47] On 29 November, the Newport Beach Police Department announced that it was conducting an active investigation into "an alleged relationship between professional basketball player Josh Giddey and a female minor".

[53] When the Newport Beach Police Department became involved with the investigation, NBA commissioner Adam Silver stated that he would take a "backseat" to allow for law enforcement to do their due diligence.

[54][55] On 17 January 2024, Newport Beach Police Department announced that Giddey would not be facing any charges, due to there being no corroborating evidence.

[55][56] Allred stated that she was unsurprised that the case had been dropped, as the girl and her parents had declined to speak to law enforcement to protect their privacy.