He went on to win a silver medal at the Gee-Bee Tournament in Helsinki and compete at the 2011 AIBA World Boxing Championships in Baku where he lost to eventual champion Everton Lopes in the second round.

Men's light-welterweight (64 kg)[3] Horn made his professional debut in Melbourne on 1 March 2013, winning by second-round technical knockout.

In just his seventh pro bout he outpointed two-time world title challenger and former IBF #1 contender Naoufel Ben Rabah.

On 10 December 2016, Horn faced former IBO welterweight champion Ali Funeka, whom he stopped in the sixth round.

With Top Rank promoter Bob Arum in attendance, his win set up a potential showdown with eight-division world champion Manny Pacquiao.

On 26 February, Pacquiao and Amir Khan announced that they had reached an agreement to fight, leaving Horn without an opponent.

[16] Pacquiao claimed Horn got away with numerous dirty tactics in the fight, using illegal blows including elbows and headbutts.

[17] In response to a formal request by the Philippine Games and Amusements Board, the WBO agreed to review the fight between Pacquiao and Horn.

It was scored round-by-round by five anonymous judges, but the WBO stated that they do not have the power to reverse the original result of the fight.

Corcoran's corner decided to throw in the towel in the eleventh round, thus Horn retained the title via TKO.

[31] In his youth, Horn had been a victim of bullying and cited this as the reason he started boxing, initially as a means to defend himself.