Born and raised in rural New South Wales, Brownless first played football for his local team Jerilderie when it was part of the Murray Football League. He boarded for three years at Assumption College, one of Australia's most renowned "football nurseries", that is, a school which has a strong tradition of producing footballers who have gone on to play in the AFL. In 1984, his final year at Assumption, Brownless kicked 155 goals. Geelong asked Brownless to come and train, but instead Brownless chose to return home to Jerilderie to catch up with family and friends and play a season of senior football before going to Geelong. During the 1985 season, he kicked 148 goals for the Jerilderie senior team while working as a farmhand. Jerilderie made it to the Grand Final but lost. Brownless kicked six of the team's nine goals despite having injured an ankle in the semi-final.
Brownless moved to Geelong in time for the 1986 VFL season and began working as a groundsman at Kardinia Park. He said that playing for Geelong was a natural progression for him because of the town's rural atmosphere:
It's really like a big country town and the club's a lot like a country club. You're all behind one another and you're all pretty close. It's a good feeling.
Brownless made his senior VFL debut in Round 1, 1986 and quickly made a name for himself as a strong full-forward, winning the Cats' Best First Year Player award that year. He went on to play 198 games and kick 441 goals, putting him 4th in Geelong's all-time goalkicking list, behind Gary Ablett (1021), Doug Wade (834) and Steve Johnson (443).
One of Brownless' fondest memories was booting a goal after the siren against Footscray in the 1994 qualifying final. With Geelong trailing by one point, he marked the ball within scoring range with just seconds left and booted a goal to send Geelong through to the next stage of the finals. While the Cats eventually made it to the Grand Final, they lost that match and it was a sad tale of Brownless' career that he played in four losing Grand Final teams with Geelong.
Brownless retired in 1997 at 30 years of age.
Billy Brownless has the record for most goals in a game at the Gabba, with 11.
Once Brownless retired from football he moved into the media, appearing regularly on The Footy Show. On The Footy Show he appeared in "The House of Bulger" segment as the family's matriarch Joybell Bulger.
In 1992 he became the inaugural winner of the Mirrool Silo Kick in the Riverina, kicking an Aussie Rules football 32 metres plus over the Mirrool Silos.
In May 2008, Bill Brownless was added as a regular on the Thursday night AFL Footy Show to replace an incapacitated Sam Newman, but was relegated to guest panelist upon Newman's return. Since 2009 Brownless has once again became a regular alongside Shane Crawford to replace Trevor Marmalade, Alternating each week with Crawford & since 2012, Matthew Lloyd.
Semi-Final: South Australia 12.8 (80) defeated New South Wales 8.11 (59), at Football Park, 3 March 1988 3rd Play-Off: New South Wales 10.8 (68) defeated Western Australia 9.12 (66), at Football Park, 5 March 1988