He left Govan High School aged eighteen, and worked in a variety of jobs including as a retail manager and a holiday rep in Tenerife before becoming a parliamentary case worker for Anne McLaughlin.

[11][12] In July 2017 McDonald introduced a Private Members' Bill to ban unpaid trial shifts for workers.

[27] McDonald has been a long-time supporter of transgender rights, and reforming the Gender Recognition Act (GRA) in Scotland to make it easier for people to self-identify.

He said in November 2019 that "I've always stood up for trans rights and I always will – there's no chance I'll desert a community that's integral to our wider movement for equality".

[33] In September 2021, McDonald said he was troubled by Scotland's relationship with China, which he claimed "opened the doorway to misinformation from Beijing".

McDonald is gay, and on 19 May 2015 gathered with other LGBT SNP MPs to campaign for a "Yes" vote in the Irish referendum on same-sex marriage, being held three days later.