During the 2007 collegiate offseason, he played for the Long Island Rough Riders in the fourth division USL Premier Development League.

In February 2009, he was drafted in the third round (34th overall) of the 2009 MLS SuperDraft by Toronto FC; however, Grella decided against signing for the Canadian club, and instead tried his luck at playing in Europe.

Replacing the injured Jermaine Beckford in the starting lineup for the visit of Stockport County to Elland Road on September 5, 2009, he scored his first league goal eight minutes into the game, curling in an opportunistic effort from the edge of the penalty area.

Grella's fifth goal of the season for Leeds came in Football League Trophy Northern Area final second leg against Carlisle United.

[7] After coming on as a first-half substitute against Norwich City, Grella was given a start against Swindon Town after Luciano Becchio's head injury ruled him out.

[8] To help maintain match fitness, Grella played 90 minutes in Leeds' 3–1 behind-closed-doors friendly win against Middlesbrough on September 14, scoring a brace in the process.

[13] Grella made his debut as a second-half substitute against Exeter City, he started the next game against Charlton Athletic, and scored his first goal for the club.

[21] On January 20, 2011, Scottish Premier League side Motherwell announced they had signed Grella on loan until the end of the 2010–11 season.

[23] This spell ended on April 12, after Grella decided he was being overlooked since Paul Hart took charge, despite making five starts in seven games.

On November 8, 2011, on his first full start for the club, Grella scored four goals in a 6–0 victory over Bournemouth in the Football League Trophy; however, once again, he failed to establish himself in Brentford boss Uwe Rosler's plans.

This win helped to ensure League 1 safety for the Shakers and gave Notts County play-off hopes a huge blow.

He made his competitive debut for the club on August 14, in a League Cup first-round match against Derby County, also scoring in their 5–5 eventual victory.

[41] On May 17, 2014, he scored his first goal for the club in a 2–0 victory over the Atlanta Silverbacks converting a penalty kick to help Carolina to a 1–0 lead.

[42] On May 31, 2014, before a regular season record crowd at WakeMed Soccer Park, Grella scored the opening goal in a 2–0 victory over the Tampa Bay Rowdies.

[43] On February 17, 2015, Grella signed with New York Red Bulls of Major League Soccer after a successful trial spell.

On October 18, 2015, in a 4–1 victory over Philadelphia Union, Grella scored seven seconds into the match, the fastest goal in MLS history,[citation needed] dislodging former Red Bull Tim Cahill.

Grella added another goal in the match, helping New York clinch the Eastern Conference regular season title.

[50] After a breakout 2015 season, supporters began calling him "Grelladinho" due to his style of play, flashy goals and ability to take on defenders with skillful dribbling.

[52] On April 24, 2016, Grella scored his second goal of the season and assisted on another in helping New York to a 3–2 victory over Orlando City SC.

[55] After an injury-shortened 2017 season, New York declined Grella's 2018 contract option and he entered the 2017 MLS Re-Entry Draft, held on December 15, 2017.

He was selected in the first round by Colorado Rapids and immediately traded to Columbus Crew SC in exchange for a second-round pick in the 2019 MLS SuperDraft.

[56] Grella made his debut for Columbus on April 28, 2018, appearing as a substitute in the 65th minute and scoring the game-winning goal in a 2–1 win at home.