He grew up in Cobden, Victoria before boarding at Melbourne Grammar School on a cricket scholarship. He is the younger brother and teammate of Jackson Merrett. He is sometimes referred to as "Zerrett" or "Junior" to easily differentiate between the brothers. Merrett made his debut in the opening round of the 2014 AFL season against North Melbourne. He was named as the substitute and came on late in the game, replacing Martin Gleeson. He was rewarded with a nomination for the 2014 AFL Rising Star after he collected 22 disposals, three tackles, and two goals in the round 11 win against Richmond. The 2015 AFL season saw Merrett spend more time in the midfield. In August, following Essendon's 87-point loss to the Western Bulldogs in round 18, it was announced that he would miss the remainder of the season with a major stress fracture in his foot. In round 21, 2016, he became the third youngest match day captain in Essendon history when he led the side against at Etihad Stadium. Under his leadership, they won just their second match for the season, defeating the Suns by six points. He won the W.S. Crichton Medal as the club's best and fairest player for 2016, becoming the youngest player in 35 years to win the award. A magnificent season was capped off by polling 19 votes in the 2016 Brownlow Medal. This was the third most votes for a player whose team won three or less games, and the most for a Bombers player since Jobe Watson polled 30 votes. Merrett was named vice-captain for the 2017 AFL season. He had another fantastic season collecting 659 disposals, receiving his first selection in the All-Australian team and being named vice-captain in the 22under22 team for the second year in a row. His 2018 season failed to reach the heights of his previous, after being concussed in the opening round against the Adelaide Crows. He was continuously tagged throughout the year, but showed signs of improvement as Essendon worked towards a resurgence. In January of the 2019 pre-season Merrett injured his ankle resulting in the use of a moon boot for 2 weeks however he was available for selection in round 1. Merrett went on to return to near his best footy during the 2019 season capped off with a 2nd club best and fairest award beating new Essendon recruit Dylan Shiel.
