McNab has achieved considerable success in chess over the years, with 8th place in the 1989 European Chess Championship and 3rd place in the 1990/91 Hastings Premier tournament among his notable results. He has also represented Scotland in ten Chess Olympiads between 1980 and 2002, with a +20 −20 =39 performance record.
Apart from playing, McNab has also contributed to many chess publications. One of his most notable works is a book on the Najdorf Variation of the Sicilian Defence, which is an important reference in chess literature.
In the 1984 Great Britain Chess Solving Championship, he secured 1st place, showcasing his aptitude not just for competitive play but also problem-solving. McNab has won the World Chess Solving Championship twice, once as part of the British team in 1992 and individually in 1995.
Interestingly, McNab is also a mathematician. He read Mathematics at Jesus College, Oxford, and has worked at the Department of Mathematical Sciences, University of Bath.