Heppell began playing football with Leongatha Football Club, but played for Gippsland Power in the TAC Cup competition for the 2009 and 2010 seasons. Named as Gippsland's captain for the 2010 season, Heppell also played for the Victorian Country side at the 2010 AFL National Under 18 Championships, where he was named in the All-Australian team. At 189 cm, Heppell played as a "line-breaking defender" for the first half of the 2010 season, while moving into the midfield for the second half of the season.
Heppell finished strongly in the Morrish Medal count, which is awarded to the best and fairest player in the TAC Cup regular season, eventually tying for the award with Jackson Sketcher of the Sandringham Dragons, after achieving the maximum votes possible from rounds 13 to 17. Despite finishing the season sixth, both Gippsland and Heppell had an excellent finals series, with Heppell having 36 disposals in the elimination final and being best on ground in the preliminary final, and Gippsland made the TAC Cup Grand Final. Heppell, in a stark contrast to the rest of his season, had a relatively poor game, gathering only 20 touches, as the Power were outclassed by Calder Cannons to the tune of 56 points.
Despite his performance in the Grand Final, Heppell was an impressive performer at the AFL Draft Camp, and was still touted by many as being a potential top five draft pick in the 2010 AFL Draft, with some even claiming he would go as high as top three. Most speculation suggested that the Brisbane Lions would select Heppell, who with his "silky" skills and raking left foot, models his style on the Lions' own Josh Drummond.
In the week before the draft, however, a rumour, which Heppell denied, began circulating that he was suffering from groin problems. Due to this, Essendon, which had the eighth selection, became interested in him, and Heppell spoke to new Essendon coach, James Hird, days before the draft. Hird, who claimed he had been interested in Heppell for months, believed that Heppell would be selected well before Essendon had the chance to draft him, but when Heppell did slide back due to groin concerns, Essendon were only too happy to select Heppell, who had supported the Bombers as a child and had "idolised" Hird when he was growing up.
Heppell was awarded the first NAB Rising Star nomination for 2011 after recording 20 possessions in his debut AFL match, against the Western Bulldogs in round 1 of the 2011 AFL season.
In his first season, Heppell played every match of the home and away season and was described as a major contributor in helping Essendon make the finals. Throughout the year, and leading into the vote count, Heppell was widely considered to be the favourite for the Ron Evans Medal, awarded to the Rising Star of each season. He won the Medal, polling 44 of a possible 45 votes and became the first Essendon player to win the award. Heppell described winning the Rising Star Award as "a dream come true".
In 2012 and 2013 Heppell's rapid rise continued, becoming one of Essendon's most important players. In 2013 he was appointed into Essendon's leadership group, in what was only his 3rd season. He became widely thought of as Jobe Watson's heir apparent to the captaincy.
In 2014, Heppell continued on the upward climb. Whilst already being accepted as a very good player and a future champion, Heppell broke into the elite category. When captain and 2012 Brownlow Medalist Jobe Watson went down with injury midseason, Heppell was the man to step up, and took his game to another level on the way to his first All-Australian guernsey and his first W. S. Crichton Medal. Heppell was also mentioned in the media as being in contention for the 2014 Brownlow Medal, and polled 14 votes. Matt Priddis took out the award with 26 votes.