She had a very distinctive, sketchy style, which was well suited to situations full of action. I am particularly fond of her cover for the 1960s Armada printing of I Carried The Horn, which I think wonderfully captures the awful tension of the moment. Mary Gernat was good at capturing that moment, if not always so good at anatomical accuracy!
You would think, from the many paperback pony book covers she did that Mary Gernat was responsible for complete illustrations, but I have been able to find only three pony books for which she provided all the illustrations: Diana Pullein-Thompson's Janet Must Ride, Dorian William's Wendy Wins a Pony and Primrose Cumming's Penny and Pegasus. It's interesting seeing how her style developed in the seven years between the two:
Wendy Wins a Pony is a much more conventional, painterly production than Penny, which is in the instantly recognisable Gernat style.
Her covers for Dragon were even simpler than the ones for Armada: most of the backgrounds are simple blocks of colour, and there is much less background detail, though all the usual action and vigour.
Tracking down bibliographical information hasn’t been straightforward either: the fact she produced covers rather than internal illustrations means it is particularly difficult to track down exactly what she did, as for many books Armada and Dragon tended to keep the original internal illustrations, and it’s the internal illustrator who tends to be listed at the copyright libraries, and rarely a separate cover artist.
Her covers are often confused with those of Peter Archer, another staff artist for Collins/Armada, so it’s worth checking if it’s important to you who did what. The two styles are quite similar, and the cover artist is not always credited, or even, in the case of Monica Edwards’ Cargo of Horses, credited to the wrong artist: although the artist credit is Peter Archer, the style looks much more like Mary Gernat. Tracking titles down then can sometimes be a case of simply following your instincts.