[2] His paternal grandparents moved from Sparta, Laconia, to Australia and Kambosos has the famous Spartan war cry "Never retreat, never surrender" tattooed on his body in homage to his heritage.

[5] Kambosos quickly dropped the excess weight and was placed in the Cronulla Sharks development squad where he was coached by Ricky Stuart but found himself at a crossroad as a teenager when he was forced to choose between boxing and rugby league.

[7] Kambosos made his professional boxing debut in May 2013, aged 19, when he faced Filipino fighter Jayson Mac Gura and was victorious via a second-round technical knockout.

[11] Five months later, he knocked out Krai Setthaphon in the ninth round and won the WBA Oceania and IBF Pan Pacific titles in the lightweight division.

[17] On 19 January 2019, Kambosos defeated Rey Perez at The MGM Grand in Las Vegas on the undercard of Manny Pacquiao vs Adrien Broner via unanimous decision in his second fight on US soil.

[18] On 7 June in Athens, Kambosos returned to his native land of Greece and knocked out Venezuelan 11–2 (9 KOs) Richard Pena in round six in front of the packed out Galatsi Olympic Hall.

[19][20] On 31 October 2020, Kambosos defeated former IBF featherweight champion Lee Selby by split decision at The SSE Arena in London.

[23] On 9 January 2021, the IBF ordered undefeated unified lightweight champion Teófimo López to defend his titles against Kambosos, their number one contender and mandatory challenger.

[30] On 5 June 2022, Kambosos Jr. and Devin Haney clashed at Marvel Stadium in Melbourne, Australia to determine the 1st undisputed lightweight champion of the four-belt era.