In Round 8 of the 2014 NRL season, Littlejohn made his NRL debut for the Sea Eagles against the Canberra Raiders.[9][10] He made 5 appearances in the halves over the course of the season.
2015
On 3 August, Littlejohn signed a 1-year contract with the Wests Tigers starting in 2016.[11]
2016
In the absence of regular halfback Luke Brooks, Littlejohn made his debut for Wests Tigers in round 1 against the New Zealand Warriors. He made 2 more similar appearances towards the end of the season.
2017
For the opening three games of the season, Littlejohn was named on the bench and played hooker when he took the field, covering for injuries to Jacob Liddle and Matt Ballin.
After an injury to halfback Luke Brooks, Littlejohn returned to the starting team in round 5. Coach Andrew Webster said, "He was one of our real leaders during pre-season training, the boys love having him around and enjoy playing with him. I’m extremely confident Jack will get the job done.".[12] On 17 November 2017, Littlejohn signed a two-year contract with English side Salford after being told by The Wests Tigers that his contract would not be renewed for the following season.[13][14]