Trengove was selected by Melbourne, with the second pick, in the 2009 AFL Draft on 26 November 2009. On 27 March 2010, he was presented with the Number 9 jersey by former Melbourne captain and games record holder, David Neitz. He made his AFL debut for the Demons in Round 1, 2010, alongside 2009 AFL Draft Number 1 pick Tom Scully.
Prince Alfred College and Sturt Football Club
Trengrove was born and grew up in Naracoorte in South Australia near the Victorian border in the south-east of South Australia.
He worked his way quickly through the junior ranks with Kybybolite and then moved to Adelaide to attend school at Prince Alfred College. While completing high school Trengove played in the SANFL, with the Sturt Football Club. He played the last half of the season with the senior Sturt side while juggling college football with Prince Alfred College. Trengove also captained the South Australian state team, in the 2009 AFL National Under 18 Championships in which he starred earning a spot in the All Australian Team. Trengove had a best-on-ground performance in the preliminary final against Glenelg, including a match-saving mark in defence.
At the start of the 2009 season, the Number 1 pick for the upcoming 2009 AFL Draft was widely considered to be potentially either of VictoriansTom Scully and John Butcher. However due to Trengove's consistent performances for Sturt, his potential to be a high pick in the draft grew as the year went on. Ultimately with the Melbourne Football Club having the first two overall selections of the 2009 Draft, it was accepted that the Demons would select Scully and Trengove, yet the order in which the club would take the two juniors was of some speculation. It was reported that Melbourne would select Trengove with the first overall selection, but Melbourne in the end decided to select Tom Scully at number one and took Trengove with the second overall selection.
Trengove made his senior debut for Melbourne on 27 March 2010 against the Hawthorn Football Club, at the Melbourne Cricket Ground. His debut came alongside fellow Melbourne 2009 draft selection, Tom Scully. Trengove was nominated for the AFL Rising Star Award in Round 5 of the season after 24 disposals, six marks, four tackles and two goals in the club's win against the Brisbane Lions. In his second season, he finished fifth in Melbourne's best and fairest award.
In Round 7, 2011, Trengove was suspended for three weeks for a dangerous tackle against Adelaide forward Patrick Dangerfield, in a case which sparked significant debate. He tackled Dangerfield around the waist, pinned one of Dangerfield's arms and slung him to the ground, causing Dangerfield's head to hit the ground and leaving him concussed; in its appeal, which was dismissed, the club argued that an attempt to kick the ball by Dangerfield had caused both players to fall awkwardly, rather than the collision with the ground being caused by a slinging action. Many players, commentators and coaches spoke out against the suspension, including several Melbourne team-mates who were later punished for voicing their disapproval via Twitter.
Trengove was announced as co-captain of the Demons alongside Jack Grimes on 3 February 2012. Playing in Round 1 in 2012 he became, at 20 years and 181 days old, the youngest club captain in VFL/AFL history. With a decline in Melbourne's fortunes came a stall in his form, averaging less disposals per game than in either of his first two seasons, despite playing all 22 games for the first time in his career. He finished eighth in their best and fairest. His pre-season was hampered by a stress fracture in his foot. It hampered his fitness levels leading into the 2013 season and his form suffered further as a result, and talks of relieving him of the captaincy began. In October, Trengove confirmed that he was stepping down as captain so he could focus on his playing form.
After playing the first two rounds of the 2014 season, Trengove missed the remainder of the season due to problems and break of his navicular bone. On October 9, 2014, during the trade period, the Herald Sun reported that he was linked to Richmond in exchange for pick 12 in the 2014 National Draft. He was reluctant to the trade and expressed his desire to stay at Melbourne, but stated that he would not block the move if it was fulfilled. The trade was set to be finalised, but a medical report was requested by Richmond, where it was discovered that he had reinjured his navicular bone on October 14, and the trade was ruled out. There was uncertainty surrounding his 2015 season and whether he could make a return, before he confirmed to Melbourne radio station, Triple M, in April 2015 that he was not going to play at all in 2015, and would instead focus on 2016.