[3][4] Johnson's first job after leaving university was working as a constituency caseworker for Edinburgh South's MP Nigel Griffiths.

[6] In January 2014, Johnson was selected by party members as a candidate for Edinburgh Southern prior to the 2016 Scottish Parliament election.

He challenged the party's Brexit policies, stating in his resignation letter he thought Labour should endorse a second referendum and campaign for remaining in the European Union.

[14] In 2018, Johnson lodged a private member's bill which aimed to protect workers selling age restricted products.

The Bill aimed to address increasing violence and abuse that shopworkers face, by introducing a new offence to protect workers and deter potential offenders.

[19] The aim of the bill is to ensure restraint and seclusion of children and young people in schools is only used as a last resort, where there is an immediate risk of harm and using appropriate methods.

[4] In 2017, Johnson revealed to Scottish Parliament that he had been diagnosed with attention deficit hyperactivity disorder (ADHD) late in life at the age of 35.