After a successful junior career he was selected at age 19 for Australia's 1995 Rugby World Cup squad, making his debut against Canada and scoring a try, followed by another 2 against Romania. By 1996 he was a more regular feature in the starting test side before becoming a firm test player for the 1999 Rugby World Cup win and the back to back Tri Nations wins of 2000 and 2001. He was also a capable goal kicker, kicking 18 penalties and 20 conversions in his test career. Roff scored the intercept try from Jonny Wilkinson's pass in the second British Lions Test in 2001. This turned the test in Australia's favour before they went on to win the game with Roff scoring a second try, squaring the series at 1-1. They went on to win the series 2–1. Until March 2007 he was the top try scorer in Super Rugby before he was overtaken by Doug Howlett. He also holds the record for most tries in a Super 12 season, scoring 15 in 1997. Midway through the super 12 season of 2004 he announced that he would retire at the end of the domestic international season. He was just 29 years old and had spent 10 years playing Test rugby and amassed 86 test caps and 244 points. He farewelled the Brumbies in 2004 as the team took the Super 12 Final.
Roff matriculated at the University of Oxford, UK in October 2006 to read Philosophy, Politics, and Economics at Harris Manchester College. He was also involved in the Oxford Union. He also made a return to amateur rugby by playing for the Oxford University, colloquially known as the Dark Blues, representing the Blues in their traditional fixture against Cambridge at Twickenham Stadium on 12 December 2006. Oxford lost this match 15–6. On 6 December 2007, he captained the Blues to their third successive Varsity loss, losing 22–16 to Cambridge, before hanging up his boots for the last time.
Later career
In June 2012, the University of Canberra Union announced that Roff had been appointed as its CEO with effect from July 2012, noting that Roff was leaving a management position with Lifeline Australia and had previously been a consultant with The Nous Group.
Other
In January 2007, he won the "United Kingdom-based Young Australian of the Year for 2007" for his services to Australian Rugby in general. In 2018, Joe joined Canberra social mixed netball team 'A New Hope'. The team was runner up in the division 3 finals. He was voted Most Valuable Player for the season. During his years with the Wallabies, the rhyming slang term "Joe Roffie" gradually worked its way into the Australian vernacular, being an abbreviation for coffee.