Born and raised in Perth, Australia, Hendriks began playing tee-ball when he was 5 rather than the traditional Australian summer game of cricket, before transitioning to baseball at the age of ten. He also played Australian rules football. and at one stage made both the All Australian football and All Australian baseball teams in the same year. Hendriks continued playing both baseball and football through high school at Sacred Heart College in his hometown of Perth.

Hendriks' grandfather and father both played football in the Western Australian Football League and Liam was eligible to be drafted by the West Coast Eagles under the Father/Son draft rule had he decided to pursue a career in football. At sixteen Hendriks was forced to make a decision as the two sports began to clash. Deciding on baseball he was signed by the Minnesota Twins on his eighteenth birthday.

Hendriks debuted for the Twins in 2007 pitching for the Gulf Coast League Twins. He led the team in strikeouts with 52. He was also named as a Twins Top 50 prospect. He pitched for the Perth Heat in the 2008 Claxton Shield and went 3–0 with a 1.90 ERA and 25 K's, a competition high. Liam was then added to the Australian national team for the 2008 Final Olympic Qualification Tournament pitching five innings of work while only allowing one unearned run alongside two hits and a walk while striking out six. Hendriks underwent back surgery that prevented him from playing in the minors in the 2008 season, but made the 2009 World Baseball Classic roster, the youngest Australian player in the Classic.

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