Dr. Mark Williams was an icthyologist who held a high position at the Instituto de Biologia Maritima. Dr. Williams, who preferred to be called by his first name Mark, was a longtime friend of fellow icthyologist Dr. David Reed and his girlfriend Kay Lawrence. Mark had helped Kay through college and given her a job as a research assistant, and her work had played a major role in his obtaining such a lofty position within the Institute. When Dr. Carl Maia requested help and funds for an expedition to find the rest of a strange fossilized skeletal hand, Mark saw the tremendous PR value of such a fantastic find and leapt at the chance to fund Carl's second team. He went along too, of course, and quickly proved to be a fairly volatile individual, disapproving of the derelict Rita and becoming extremely frustrated after they failed to find the rest of the skeleton.
When the part visited the Black Lagoon on a hunch of David's that the they could find the rest of the fossil there, they found instead the Gill Man. The monster quickly became too much for the small expedition to handle. Mark wanted the proof, determined to capture the Gill Man dead or alive; Mark, constantly wielding a speargun, seemed to prefer dead. His multiple attempts to kill the creature resulted in the Gill Man developing a personal vendetta of sorts against Mark. When David was attempting to move the tree that was blocking the lagoon entrance so they could leave, he was attacked by the Gill Man, but Mark came to his rescue. The two struggled, but the Gill Man emerged the victor when he tore the air hose of Mark's scuba gear, drowning the scientist.
In the 1977 Carl Dreadstone novelization of Creature From the Black Lagoon, Mark is renamed Bruno Gebhardt and is a naturalized Brazilian man of German ancestry. Rather than perishing by drowning, he dies when the Gill Man, who is quite large and weighs thirty tons in this version of the story, lands on him and crushes him.