In 1989 Hancock made his debut for the Queensland Maroons, becoming the then-youngest footballer of either state to play in a State of Origin series and scored two tries in the 36-6 win over NSW at Lang Park. He then made his Test debut on the 1989 mid-season tour of New Zealand while still a teenager. Having represented his state that year, proving himself a top-class winger with pace, strength and elusiveness, he played in all six matches on the New Zealand tour, scoring three tries. He played in all three Tests against the Kiwis and scored his first try in test football in Australia's 8-0 win in the second test at the Rotorua International Stadium on 16 July. He was known for a wide-legged running style that allowed him to change direction sharply and sometimes several times as he approached opposition defences. Hancock was named the 1989 Brisbane Broncos season's rookie of the year.
In 1990 Hancock missed the one-off Test against France with a hamstring injury, but regained his spot on the wing for the test against New Zealand in Wellington where he scored a try in Australia's 24-6 win. He was then selected for the 1990 Kangaroo Tour of Great Britain and France at the end of the 1990 NSWRL season. Unfortunately, an ankle injury suffered in Australia’s 19-14 loss to Great Britain in the first test loss at Wembley, London limited him to just six games on the English leg of the tour and he lost his test spot to Broncos team mate Dale Shearer. The first test loss was something of a disaster for Hancock as his opposite winger Paul Eastwood crossed for two of the Lions three tries on the day. Although he returned to action, before the second Ashes series test, scoring a try in a 36-18 win over Halifax, Shearer's form was such (scoring a try in the Aussies 14-10 win at Old Trafford in the second test) that Hancock was unable to regain his test spot for the remainder of the tour.
Hancock made his second tour of New Zealand in 1993, playing all three tests of the 1993 Trans-Tasman series and scoring his 5th and final test try in the third test at Lang Park. He played in Brisbane's second consecutive premiership win in 1993, again defeating St George in the Grand Final. During the 1994 NSWRL season, Hancock played at centre for Brisbane when they lost 20-14 against Wigan in the 1994 World Club Challenge in front of a WCC record attendance of 54,220 at Brisbane's ANZ Stadium.
Michael Hancock played his last test for Australia in 1994 against France in a 58-0 win at the Parramatta Stadium in Sydney. Called into the team after injury ruled out Broncos team mate Willie Carne, Hancock didn't score in the game but made a number of breaks and almost always had the hapless French grasping at air trying to tackle him. After being selected for his second Kangaroo Tour in 1994, his test career ended when he injured his shoulder in a training mishap early in the tour. In a case of Déjà vu with 1990, he would return to the field before the tour ended, but wasn't able to regain his test place either on tour or in the following years.
The 1995 ARL season saw Hancock, along with fellow Kangaroo tourist Steve Renouf, top the Broncos' try scoring list with 15 for the season. This was the only time in his career that Hancock was the Broncos season leading try scorer.
Despite scoring a career single-season best 15 tries in 1995, Hancock was cut by Broncos coach Wayne Bennett at the beginning of the 1996 season following the emergence of youngsters such as Darren Lockyer and Michael De Vere, both of whom were as fast as Hancock, but both were also handy goal kickers. Determined that he wasn't finished as a player at the age of just 26, Hancock asked Bennett to be allowed to train with the team in the off-season in what was a successful bid to regain his contract. He repaid Bennett by going on to score 13 tries for the year in 22 games. His form also saw him regain his place on the wing for Queensland, playing from the bench in a 14-6 loss at Lang Park, though he was dropped for the rest of the 1996 Origin series which was won 3-0 by NSW.
Although his representative career ended, he savoured another title in 1998, again playing from the bench in the Broncos' 38-12 victory over Canterbury in the 1998 NRL Grand Final. Until the Grand Final against the Bulldogs, Hancock had played 25 games on the wing during the season, scoring 10 tries. For the decider, Bennett moved Hancock to the bench and started Michael De Vere on the wing.
The advent of the unlimited interchange rule prolonged Hancock’s career, with coach Wayne Bennett opting to use him as an impact player on the wing or in the Second-row. The last of the foundation players to leave the club, it was only fitting then, in his fourteenth season with the same club, that he left the Broncos after the 14-6 win against the Sydney Roosters in the 2000 NRL grand final to play out the rest of his career for Salford in England. Also in 2000 Hancock was awarded the Australian Sports Medal for his contribution to Australia's international standing in the sport of rugby league. At the time of his retirement, he held the record for most appearances for the Broncos and was number two on the club's all time top try-scorers list with 120 (second only to Steve Renouf's 142).
In 2003 Hancock was one of the first four former players inducted into the Broncos official Hall of Fame. During the 2007 season at the Broncos' 20-year anniversary celebration, the club announced a list of the 20 best players to play for them to date which included Hancock. In 2008, rugby league in Australia's centenary year, Hancock was named on the wing in the Toowoomba and South West Team of the Century.