Fenech had a stellar amateur career, which led to him representing his country at the 1984 Olympics in Los Angeles. It was that year that Fenech began his professional boxing career, beating Bobby Williams by a knockout in two in his homeland. Fenech was a fast starter: He won his first eleven bouts by knockout, and held his first fight abroad in only his fourth fight, when he beat Iliesa Manila by a knockout in two at Fiji.
He beat fringe contenders Wayne Mulholland and Rolando Navarro, both by a knockout in the fifth round, to start 1985. After those two wins, he was placed number one among the world's Bantamweights by the International Boxing Federation. Fenech only took six professional fights to become the number one contender, and he placed in fifth place all time for the boxer who got to fight for the world title the fastest after beginning his career when he challenged Shatoshi Shingaki for the world's title in only his seventh bout, displacing the second Davey Moore, and trailing Leon Spinks, Saensak Muangsuring, Pete Rademacher and Rafael Lovera. Fenech was the third fastest boxer to become a world champion, behind Muangsuring and Spinks, when he knocked out Shingaki in nine rounds in Sydney. After two non title knockout wins, he gave Shingaki a rematch and retained the crown with a knockout in three. After one more non title knockout win, Fenech had to go the distance for the first time, when he faced Jerome Coffee, retaining the title by a 15 round unanimous decision.
In 1986, Fenech had only two bouts, but they were both major fights: He won a 10 round, non title fight decision over former world Bantamweight and future world Featherweight champion Daniel Zaragoza, and he retained the world championship with a knockout in 14 over the late Steve McCrory.
1987 was a big year for Fenech. He went up in weight, and in his first fight as a Jr. Featherweight, he defeated Tony Miller by a decision. Then, the WBC's world Jr. Featherweight champion, Samart Payakarum, travelled to Australia to defend his belt against Fenech. Fenech became a two time world champion by knocking Payakarum out in four rounds. For his next defense, he met future world Jr. Bantamweight champion Greg Richardson and knocked him out in five. Then, it was former world Bantamweight champion Carlos Zarate's turn to challenge Fenech. Fenech retained his crown by a technical decision win in four rounds, and then finished his year by knocking out Osmar Avila in one round in a non title affair.
By 1988, Fenech had grown into a Featherweight, and the WBC faced him with Puerto Rico's former world Jr. Featherweight champion Victor Luvi Callejas, once again, in Sydney. Fenech became a three time world champion, joining boxing's exclusive group of fighters who have been world champs in three or more divisions, by knocking Callejas out in the 10th. round. He retained that title twice before the year was over, knocking out Tyrone Downes and Georgie Navarro, both in the fifth round.
In 1989, he beat Marcos Villasana and former Julio Cesar Chavez challenger Mario Martinez, both by decision.
After that, Fenech took one year off and came back in 1991 as a Jr. Lightweight. After beating Johnny Calhoun by a knockout in four, he tried to become a four division world champion when he met His Highness Azumah Nelson (who, apart from being a world boxing champion, also happens to be an African prince). He came close to beating Nelson on an HBO Boxing televised bout, but the fight was declared a draw after twelve rounds in Las Vegas. After beating Miguel Francia in Australia to finish 1991, HH Nelson travelled to Melbourne to offer Fenech a second title try at Jr. Lightweight, and this time, Fenech suffered his first loss, when he was knocked out in the eighth round for HH Nelson to retain the world title.
After that fight, he fought sporadically. In 1993, he was beaten in seven rounds by knockout by American former world Featherweight champion Calvin Grove, and in 1995, he beat Tialano Tovar, by a knockout in eight at New Jersey. Finally, in what would turn out to be both his last title fight and his last fight, he lost to IBF world Lightweight champion Phillip Holiday of South Africa by a knockout in the second round in 1996. He retired after the bout.
In retirement, Fenech has kept busy, and now he is the owner of a sports clothing brand that carries his name in Australia. In addition, he was inducted in 2002 into the International Boxing Hall Of Fame in Canastota, New York.
On January 5, 2004, Fenech was attacked and stabbed by four men in Sydney, receiving facial cuts that required plastic surgery.
Fenech retired with a record of 27 wins, 3 losses and 1 draw, with 20 wins by knockout.