The son of Peter and Carole, Slater was born in the New South Wales rural city Wagga Wagga after his parents and two elder siblings (Julian and Tracey) had emigrated from the north-eastern coast of England in 1966. They originally migrated to Launceston, Australia by boat, after deciding against moving to Canada. In Launceston, Slater's father taught agriculture and science at high school, as he had done since graduating from college. After three years in Launceston, the Slaters moved to Wagga Wagga, and Peter became a lecturer in agriculture at what is now known as the Charles Sturt University. Slater lived in Wagga Wagga for his entire childhood, eventually leaving with only fond memories. He later wrote: "Wagga Wagga was a great place to grow up, for many reasons. It was a good size — it had a population of around 50,000 when we moved there — and it was in the country, with a great Australian climate. My family was always involved in sport, so from an early age it just seemed natural for me to play any game that was on offer."

When still in primary school and aged 11, Slater was selected in the New South Wales Primary School Sports Association cricket and hockey teams, something he described as "a very big thing" and "totally unexpected". A left-inner (an attacking player who helps out defensively), he also made the state hockey team at Under–12 level in 1981, before going on to later be selected in the Under–13, ?15 and ?17 teams. Slater's mother unexpectedly left the family in 1983, when Michael was just 12 years old. He later wrote about the tough personal times that followed.

After his mother left for good, Slater's education standards slipped, with sport becoming the "only thing could focus on properly". School bullying accentuated his academic difficulties in Years 9 and 10, and he once ran home after it was suggested among fellow pupils that some bullies "were planning to get after school". As cricket and hockey began to overlap in his early teenage years, Slater slowly turned the majority of his sporting appetite towards cricket. He joined an inner-western Sydney Under–16 side over the Christmas holidays. Despite not being fond of Sydney, Slater knew that he would have to move to the metropolis if he wanted to further develop his cricketing career. After topping the batting averages in the Under–17s in the following season, Slater was subsequently chosen as captain for the New South Wales Under—16 team. The carnival was not a personal success for the captain; however, he recalls that his team performed "well".^[7]

An accident at school when he was seventeen saw Slater hurt his Achilles tendon. He played a couple of hockey games following the accident, but had to limp off the field and have consequent surgery in the lead-up to the Under–17 national cricket carnival. Soon after, Slater was informed that, because of his injury, his "dream of playing cricket for Australia was over".^[8] After an operation, though, he surprisingly returned to cricket and was selected in the Under–19 state team for the national championships in Brisbane. After an injury to the captain, Slater once again captained his state, although he and his team under-performed.^[9] He returned as vice-captain the following year for the Under–19 carnival in Canberra, and scored a century in the opening match. In a victorious final against Victoria, Slater scored another century, becoming one of the leading run-scorers in the series.^[10]

A specialist right-handed batsman as well as a very occasional bowler