[2][3] He grew up playing rugby league in Far North Queensland while attending Mareeba State High School until the age of 15 when his older brother, Callum, introduced him to basketball.

[4] As a result, Baynes decided to focus on pursuing a career in basketball and subsequently quit rugby league.

Shortly after high school graduation, he joined the Australian Institute of Sport in 2004–05 and accepted a college scholarship offer from Tony Bennett to play for Washington State University in 2006.

[8] On 15 July 2010, Baynes signed a two-year deal with EWE Baskets Oldenburg of the German Basketball Bundesliga.

[16] He made his first NBA start in Game 4 of the Spurs' first-round playoff match-up against the Los Angeles Lakers, and was tasked with defending Dwight Howard.

[21] On 6 May 2014, he recorded playoff career-high numbers of 10 points and seven rebounds in a 116–92 win over the Portland Trail Blazers in Game 1 of the Western Conference semi-finals.

[30] In a game against the Charlotte Hornets, Baynes injured Celtics teammate and star point guard Kyrie Irving.

[31] In the Celtics' regular season finale on 11 April 2018, Baynes led a short-handed Boston lineup with a career-best 26 points and 14 rebounds in a 110–97 win over the Brooklyn Nets.

[37] On 6 July 2019, Baynes, along with the draft rights to Ty Jerome, was traded to the Phoenix Suns for a 2020 protected first-round pick.

[38] After Deandre Ayton was suspended for 25 games due to diuretic usage on 24 October, Baynes was promoted to being the team's starting center during that period of time.

[39] On 6 March 2020, Baynes scored 37 points and hit nine three-pointers, both career-highs, along with 16 rebounds, in a 127–117 victory over the Portland Trail Blazers.

[46] After recovering from the virus, Baynes was later diagnosed with a right knee contusion, leaving him out of action for the rest of the resumed regular season.

Baynes was also part of the national team for the 2020 Summer Olympics in Tokyo, but a spinal cord injury ruled him out for the second half of the tournament as the Boomers went on to win the bronze medal.