Born in Merthyr Tydfil, he played for his school, Bishop Hedley High School,[citation needed] and then club rugby for Merthyr and Pontypridd, being voted Pontypridd's Player of the Year in 2001–02.
During his time at Merthyr, Sidoli studied for a BSc in computer science at Cardiff University.
After good performances in the 2003 Rugby World Cup, he missed most of the 2004 Six Nations Championship with a groin injury.
He was in the starting line-up for all Wales matches in the Grand Slam-winning team of 2005, and scored his first international try against Italy during that tournament.
His brother, Wales under-21 international Peter Sidoli, nationalised to play for their father's native Italy, but was never selected.