That sounds familiar so I looked it up[0]. I used Mark Williams C compiler on the Atari ST--eventually settling on Megamax C as it ran better on my small floppy-based machine.

Computing was a smaller world back then, the company was founded by Robert Swartz (father of Aaron Swartz) and named after his father William Mark Swartz.

[0] https://en.wikipedia.org/wiki/Mark_Williams_Company