Laidlaw was born in 1960 and raised in Laguna Beach, California and attended the University of Oregon. Here he tried, and was discouraged by punch card computer programming. He wrote short stories and his first novel, Dad's Nuke, which was published in 1985. This was followed by several more novels over the next decade, but he worked as a legal secretary in San Francisco for a living.
Laidlaw had played computer and arcade games, but was not intrigued. It was not until Myst that his perception of these games changed. He obsessed over Myst and bought a new computer so that he could play it at his San Francisco home. With his new-found interest, he wrote The Third Force (1996), a tie-in novel based on the world created by the Gadget computer game. His favorite PC game of all time is Thief: The Dark Project (interview)
Working with game designers led him to feel that he wanted to help design an actual game. He joined Valve Software while they were developing Half-Life ( 1998) and worked on the game's story and level design. At Valve, he later worked on Half-Life's expansions, Half-Life 2 and its future episodes.
In January 2016 Marc announced via an email exchange with a fan that he had retired from Valve (his career at the company spanning 18 years) and was now looking to return to his own writing.