As Maxwell remained injured, McDonald found herself playing starting scrum half in the disrupted 2020 Women's Six Nations Championship - her first championship performance. Her coach Philip Doyle said of the decision, "“It was always Mairi who was going to start this one. She showed a very composed performance against Spain off the bench which really impressed us as a coaching group."" She started in the matches against Ireland, which was a defeat (18-14) and France.