Witherden was a successful junior athlete in basketball and cricket but his biggest aspiration was to play in the AFL. He loved football so much that his mother forged his age in order to go to Auskick a year early. He regularly played football above his age group as a junior for Barwon Heads in the Bellarine Football League, mainly playing as a midfielder. In 2009 as a 10 year old he played under 12s and under 14s for Barwon Heads then he just played under 14s in 2010. In 2011 he moved to St Joseph's Football & Netball Club where he played two seasons of under 14s. Alex moved up to under 16s for 2013 and in 2014 he was named in the Vic Country team for the Under 16 National Championships. Witherden had arthroscopes on both hips at the end of the 2014 season. 2015 was when Witherden made his debut for the Geelong Falcons in the TAC Cup as a bottom age player, he played 12 games and kicked 5 goals. He also played in the AFL Academy game on AFL Grand Final, drawing strong praise from coach Brenton Sanderson. Witherden's draft year started off strongly in 2016, he was named the captain of the Geelong College 1st XVIII football team after an impressive season in 2015. Witherden played two Academy games and two Falcons games where he averaged 23 disposals and five marks per game in the TAC Cup, playing mostly as a skilful defender. While playing for Geelong College in early May, they were losing by about 90 points, as Witherden was sick before the game so he was just playing forward however he then decided put himself into the midfield for a centre bounce. He broke his leg after a tackle from fellow AFL prospect Tim Taranto who was playing for St Kevin's. Witherden's leg was in an awkward position and Taranto came down on it with his full body weight, Alex was in severe pain and had to be stretchered off the ground. He broke one of the bones in his right leg in four places, cracked another one and tore the syndesmotic ligaments, which connect them. The injury meant Witherden would finish the most important year of any junior footballer having played just two games of TAC Cup, he was also forced to miss the Under 18 National Championships, the Under 18 All Stars match and was in doubt for the National Draft Combine, where he received an invitation. Witherden focused on his rehab, he rode an electric scooter around school as it was the best way to keep weight off his leg, he also hit the Geelong Falcons gym after dropping down to 75kg during his injury, his weight improved to 80kg and improved his diet, cutting down on his favourite, McDonald's frozen Coke plus ate less bread. Where at one time he would have three or four sandwiches in his lunchbox, he then would opt for a chicken salad. He took advice during the setback from fellow draftee and friend Jy Simpkin, who broke his right leg a month earlier and Geelong Cats player Darcy Lang who suffered a similar injury.
AFL career
He was drafted by Brisbane with pick 23 in the 2016 national draft. He made his debut in the loss to at the Gabba in round 14 of the 2017 season. After the thirty-one point loss to at Etihad Stadium in round 17, in which he recorded twenty-eight disposals and four tackles, he was the round nominee for the 2017 AFL Rising Star.
