In his youth, Grillo was quarter-finalist at the 2008 U.S. Junior Amateur. He got the 2009 Byron Nelson International Junior Golf Award.
Grillo turned professional in 2011 and earned playing status on the European Tour via Qualifying School. He finished 10th in his first event, the 2012 Africa Open. He finished 94th in the 2012 European Tour Order of Merit and 89th in 2013.
In February 2014, Grillo had his best finish to date on the European Tour, when he finished runner-up behind Stephen Gallacher at the Omega Dubai Desert Classic. Later in the season he finished 8th at the BMW International Open and the BMW Masters, and 11th at the Turkish Airlines Open. He ended 44th on the European Tour Race to Dubai. He also won the Visa Open de Argentina on PGA Tour Latinoamérica.
Grillo began the 2015 European Tour season with a third place at the Volvo China Open, a fifth place at the Qatar Masters, a sixth place at the Open de España and an 11th place at the Shenzhen International. He finished fourth at the DP World Tour Championship, Dubai and 40th on the Race to Dubai.
Also in 2015, Grillo lost in a five-man playoff at the Puerto Rico Open on the PGA Tour, playing on a sponsor's exemption, and finished tenth at the Barbasol Championship. In just seven starts, he earned enough to qualify for the 2015 Web.com Tour Finals. He claimed a ninth place at the Hotel Fitness Championship, a runner-up at the Small Business Connection Championship and a first place at the Web.com Tour Championship. Therefore, he finished second at the Finals money list and got a card for the 2016 PGA Tour. Since Grillo only played in seven events during the 2014–15 season, he was still considered a rookie for the 2015–16 season.
In his eighth career PGA Tour event and first as a member, Grillo won the season-opening Frys.com Open on the 2016 PGA Tour, after beating veteran Kevin Na in a sudden-death playoff. He made a 25-foot birdie putt on the 72nd hole for 3-under-par 69, which was later tied by Na. Grillo then missed a three-foot putt for victory on the first playoff hole, but sealed the win on the second hole with a birdie after Na ended up behind a tree and could only make bogey. The victory gained Grillo entry to the Masters and the 2016 PGA Championship. He also moved up to 36th in the Official World Golf Ranking.
At the 2016 major tournaments, Grillo finished 12th at the Open Championship, 13th at the PGA Championship, 17th at the Masters and 54th at the U.S. Open, with score cards of 283, 274, 292 and 293. He was runner-up at the Japan Golf Tour's ISPS Handa Global Cup. He was voted 2016 PGA Tour Rookie of the Year.2007 Pereira Iraola Cup (as low amateur at the Argentine Open)
2009 Pereira Iraola Cup (as low amateur at the Argentine Open)
2011 Terra Cotta Invitational
PGA Tour playoff record (1–1)2014 Visa Open de Argentina
CUT = missed the half-way cutMost consecutive cuts made – 6 (2015 PGA – 2017 Masters)
Longest streak of top-10s – n/a
"T" = tied
DNP = Did not play
QF, R16, R32, R64 = Round in which player lost in match play
"T" = tied
AmateurEisenhower Trophy (representing Argentina): 2008, 2010
ProfessionalWorld Cup (representing Argentina): 2013
Presidents Cup (representing the International team): 2017