Stanlake was born in Hervey Bay, Queensland and spent the first eight years of his life on the Fraser Coast. There, he and his brother played Australian rules football for the Hervey Bay Bombers, encouraged to do so by their father Warren Stanlake who played for the Footscray Football Club in the professional Victorian Football League. The family then relocated to the Gold Coast, where Billy began playing cricket for Southport-Labrador and attended The Southport School (TSS).

After switching to cricket, Stanlake was a very successful youth cricketer in both domestic and international competitions. He represented his home state of Queensland in the 2013 under-19 national championships and took fourteen wickets in three matches, including bowling figures of 6/30 and 5/40.

Stanlake played for Australia's youth team, being named in their squad for both the 2012 and 2014 Under-19 World Cups. He faced a foot injury before the 2012 World Cup and was unable to play, but although he had another foot injury ahead of the 2014 World Cup, he was able to recover in time to participate. During the tournament he had both the best bowling average and economy rate of the Australian bowlers, taking five wickets at an average of 19.40 with an economy rate of 3.34 runs per over.

During his youth career Stanlake played twelve youth One Day Internationals for Australia from 2011 to 2014. His best bowling performance came in 2011, when he was still sixteen years old, against Sri Lanka. He took four wickets for 37, taking out most of Sri Lanka's top order. Overall he took 13 wickets in youth ODIs at an average of 29.92.

Domestic debuts (2014–2016)

In the wake of his Under-19 World Cup performances, Stanlake was offered a rookie contract with the Queensland state cricket team for the 2014–15 season, but he missed the entire season due to stress fractures in his back. He instead made his debut in all three major formats of cricket (first-class, one-day and Twenty20) in the 2015–16 season.

Stanlake was named in Queensland's squad for the first time for the 2015–16 Matador BBQs One-Day Cup, and he m