Boak made his AFL debut in a 31-point victory in Round 12, 2007 against Essendon. Boak continued to rise since his debut, having 23 possessions against the Western Bulldogs in only his third match. The following week, against Richmond, Boak kicked his first ever AFL goal. Boak's greatest individual achievement came in Round 19, 2007 against Carlton when he earned a NAB Rising Star nomination for his 28 possessions and 2 goals. Boak's first season of AFL football was also memorable in the fact that he was part of the Port Adelaide team which competed in the 2007 AFL Grand Final against Geelong. Unfortunately for Boak, he was unable to become a Premiership player in his first season of AFL football as Port Adelaide were convincingly beaten in the Grand Final. He finished the year ranking third in rising star nominations in handballs.
At the start of 2008, Boak picked up where he left off, playing a greater role in the Power midfield alongside senior players such as Domenic Cassisi, Kane Cornes and Shaun Burgoyne. He played in 17 games and averaged 20.1 disposals. Boak continued to improve in 2009 winning the Gavin Wanganeen Medal and averaging 23.7 disposals for the season.
The 2010 season was Boak's best season to date with a career best average of 23 possessions per game. His season ended with a flourish when he kicked three goals in each of the final two rounds. He became the first-tagged player in the Port Adelaide midfield and finished the season with 16 Brownlow votes.
The 2011 season saw Boak play in 21 games and leading club in disposals (458), clearances (79) and inside 50s (85). He along with Jackson Trengove were jointly crowned the John Cahill Medallist, the first time that there had been a tie at the club's best and fairest award.
For the 2013 season, Boak was announced as the captain of the club, therefore replacing his number #10 guernsey with #1 as is the Port Adelaide tradition. 2013 was considered to be Boak's best AFL season up to that point, with an average of 25.3 disposals a game, and kicking a season best of 20 goals. He was named in the 2013 All-Australian team. In 2014, Boak continued where he left off from 2013, averaging a career best 26.6 disposals a game and tolling a season best 21 Brownlow votes to finish equal 4th. For the second consecutive season he was named in the 2014 All-Australian team.