During his time as Head of Serials, MacDonald influenced Doctor Who. It was ultimately up to him to approve a script and he inspected each of them for elements the BBC did not approve of. For instance, he insisted that the suicide of Maximillian Stael in Image of the Fendahl take place off screen.
MacDonald wanted to see a reduction in the slapstick present in Doctor Who. As a result of this, Anthony Read asked Robert Holmes to dampen the humour for his script, The Power of Kroll. After watching this serial, MacDonald was so unimpressed by the set design, that he ordered that designer Don Giles was never to work on Doctor Who again.
In 1977, it was MacDonald who asked Williams not to produce The Vampire Mutation as it would coincide with the BBC's adaptation of Count Dracula. This script would resurface in 1980, becoming State of Decay.