Day had early success as a professional, earning PGA Tour membership in his teens and winning an event in his third season, the HP Byron Nelson Championship.

Later he went to Hills International College, where they have a golf academy,[9] at the behest of his coach, Col Swatton, who had moved there when Kooralbyn school closed down.

Day borrowed a book about Tiger Woods from his roommate, and it inspired him to improve his golf by practicing in the early morning, at lunch-time and in the evening.

His amateur success extended to the United States, where he won the Boys 15–17 division at the 2004 Callaway World Junior Championship and was runner-up in the 2005 Porter Cup.

In 2005, Day lost in a playoff at the Greater Building Society QLD PGA Championship; a professional event on the Von Nida Tour.

[12] Day turned professional in July 2006 after winning the Green Jacket at the NEC Master of the Amateurs, signing with TaylorMade and Adidas[13] and immediately began playing PGA Tour events, principally through sponsors' exemptions.

In August 2010, Day made his first appearance at the PGA Championship, where a 66 on Saturday helped him to finish seven-under-par for the tournament and earn his first top-10 in a major.

[16] At the 2011 Masters Tournament, Day birdied the last two holes, but came up two strokes short of eventual winner Charl Schwartzel.

[19] As a result of his major performances, Day moved into the Official World Golf Ranking top-10 for the first time in his career at ninth.

He contended in a major once again at the 2013 Masters Tournament when he shot rounds of 70 and 68 to take a one-shot lead over Fred Couples going to the weekend.

[22] In February 2014, Day won his first World Golf Championship event, the WGC-Accenture Match Play Championship, after a final in which Victor Dubuisson won the last two holes to tie, then twice scrambled halves after missing greens at the extra holes before Day finally secured victory.

[23] In February 2015, Day won his third PGA Tour event and his seventh title as a pro, winning the Farmers Insurance Open with a score of 279 (−9) after prevailing in a four-way playoff over Harris English, J.

At the 2015 Open Championship at St Andrews, Day entered the final round as one of the 54-hole co-leaders and shot a bogey-free 70 to finish at 14-under-par and one stroke outside of the 3-way playoff.

[27] The following week at the RBC Canadian Open in Oakville, Canada, Day outlasted Bubba Watson and hometown favourite David Hearn to win the tournament, his second tour victory of the season and fourth overall.

Jason Day produced an exceptional performance shooting rounds of 68, 67, 66 and 67 to best his nearest competitor, Jordan Spieth, by three strokes.

[29][30] Day's hot streak continued with wins in The Barclays and the BMW Championship, two of the first three events in the FedEx Cup playoffs.

Jordan Spieth would win the Tour Championship and the FedEx Cup as well as retaking the number one world ranking.

[32] After a missed cut in the defence of his Farmers Insurance Open title, Day claimed his first win of the year and eighth overall on the PGA Tour at the Arnold Palmer Invitational, where he beat Kevin Chappell by one shot.

1 with his progression through to the semi-finals[34] before beating Louis Oosthuizen 5 & 4 in the final in Austin, Texas for his ninth PGA Tour win.

[37] In late June, Day announced that he would not be playing in the 2016 Olympic Games in Rio de Janeiro, citing concerns over the Zika virus.

It was announced that he will wear Nike clothes, shoes and gloves, following the brand's decision to stop making clubs and balls.

On 13 September 2017, after a winless season, Day split from his caddie of 11 years – his entire professional career – and 'father figure' Colin Swatton.

[39] In January, Day won the Farmers Insurance Open, at Torrey Pines for a second time, for his 11th PGA Tour win and first in over eighteen months.

Day finally claimed the victory with a birdie on the sixth extra hole, after Norén had found water with his second shot to the green.

The balloon therapy, which takes about 20–30 minutes twice a day, is supposed to help get his rib cage, hips and shoulders aligned, thereby alleviating pressure on his back.

Case in point: following an uninspiring round of even par on a rainy Thursday afternoon at the Travelers Championship, Day headed straight to the range at Williams' request.

[43] On 21 October 2019, Day won The Challenge: Japan Skins over Tiger Woods, Rory McIlroy and Hideki Matsuyama.

[46] In December, Day won the inaugural Grant Thornton Invitational mixed team tournament with playing partner Lydia Ko.

1Cancelled due to COVID-19 pandemic QF, R16, R32, R64 = Round in which player lost in match play WD = withdrew NT = No tournament "T" = Tied Note that the Championship and Invitational were discontinued from 2022.