The talented 23-year-old midfielder, who played just eight games in 2014 after battling a back injury, asked to be traded to the Lions last week in a surprise move. 

He met with the Brisbane Lions and Gold Coast but the Lions were keen to secure him. 

Christensen has played 65 games and kicked 56 goals and was a member of the Cats 2011 premiership team. 

He finished eighth in the Cats best and fairest in 2013 and looked to be emerging as a vital component of the Cats' midfield. 

Christensen joined Geelong as selection No.40 in the 2009 NAB AFL Draft. 

Lions talent acquisition and retention manager, Peter Schwab told lions.com.au Christensen was an exciting addition to the club's midfield. 

"Allen is a fantastic player who brings some good finals experience to our young list. He offers a great injection of pace into our side and will be a valuable addition," Schwab said.

The Lions received pick No.21 last week after trading two time best and fairest Joel Patfull to Greater Western Sydney.