Mark has specialized in object-oriented programming, internationalization, and text software for many years. Mark co-founded the Unicode project and has been the president of the Unicode Consortium since its incorporation. He is a principal co-author of each version of the Unicode Standard, and is responsible for many of the major Technical Reports and Standard Annexes.

Mark is the architect for ICU, the premier Unicode software library sponsored by IBM. Earlier in his career, he worked in Switzerland for several years, then returning to California to join Apple, where he co-authored the Macintosh KanjiTalk and Script Manager, and authored the Arabic and Hebrew systems. Later, he was the manager and architect for the Taligent international frameworks, and more recently was the architect for a large part of the Java international libraries. He is the author of a number of patents, mostly in internationalization. At various times he has also managed groups or departments covering text, internationalization, operating system services, Windows porting, and technical communications.