Hayes grew up in suburban Sydney and began playing junior football with Pennant Hills in the Sydney AFL. In 1998, Hayes was a standout player for NSW/ACT in the representative TAC Cup, taking out the Morrish Medal as the best player in the competition. Hayes debuted in the AFL the following year. He started well and improved throughout the year, earning an AFL Rising Star nomination in the final round of the season.
2000s
With St Kilda's current policy being that of one to rotate the captaincy to a different player each season, Hayes would be named captain ahead of 2004. After he was sworn into the role, Hayes captained St Kilda’s 2004 Wizard Home Loans Cup winning side, in what was the club's second pre-season cup win. After a strong start to the 2006 season, Hayes ruptured his anterior cruciate ligament against the Kangaroos in Round 9, requiring a minor knee reconstruction which sidelined him until the 2007 season. In just his fifth game back in the 2007 season he suffered a broken collarbone, sidelining him for three weeks, although he managed to come back and play 19 games for the year. Hayes was a co-captain and part of the Saints' 2007 leadership group – dubbed the "Young Saints", which also included Justin Koschitzke, Luke Ball and Nick Riewoldt. Additionally, others included in the leadership group were Aaron Hamill, Fraser Gehrig, Nick Dal Santo and Robert Harvey. Hayes played in St Kilda’s 2008 NAB Cup winning side, the club’s third pre-season cup win. In 2009, Hayes played his 200th AFL game in Round 9 against the Brisbane Lions at Docklands Stadium. During the match he gained 31 possessions in the Saints' 16-point victory. He finished third in the 2009 Brownlow Medal. Hayes played in 19 of the 22 matches in the 2009 home and away rounds in which St Kilda qualified in first position for the finals series, winning the club’s third minor premiership. Hayes was recognised for his excellent season with selection in the 2009 All-Australian team as a midfield player. It was his third career selection in the All-Australian team. St Kilda made the 2009 AFL Grand Final after qualifying and preliminary final wins. Hayes played in the Grand Final in which St Kilda were defeated by 12 points.
2010s
Hayes played 25 games in 2010 and received a number of accolades for his performances during the season:
Trevor Barker Award as St Kilda's best & fairest player.
2010 Norm Smith Medallist.
St Kilda's player of the finals series.
Hayes injured his left ACL early in the 2011 season and did not play for the rest of the year. Hayes played all games in 2012 and won his third Trevor Barker Medal as St Kilda's best & fairest player for the season. He had a fantastic season and made a brilliant recovery from the ACL injury from 2011. He played his 250th game in Round 9 against eventual premiers, gaining 25 disposals in a 28-point win for the Saints. On 15 July 2014, Hayes announced his retirement in front of all his teammates and family. Out of respect, his teammates wore t-shirts with the hashtag #ilovelenny. At the time of his retirement, Hayes held the all-time VFL/AFL record for most tackles in a career, having laid 1,496 tackles at an average of 5.04 per game. He held the record until 23 April 2017, when he was overtaken by Matt Priddis.
Coaching career
After working in the media for twelve months, Hayes returned home to Sydney in 2015 and joined as an assistant coach.
