Lamas was born in Santa Monica, California, in 1958, the son of Argentine actor Fernando Lamas and Norwegian American actress Arlene Dahl. He is the stepson of swimmer and film star Esther Williams, who married his father when Lamas was 11 years old. Both Williams and Dahl were also best friends of actress Jane Wyman, who knew him since birth, and she would later play along with Lamas on Falcon Crest. He was brought up in Pacific Palisades, California, and moved to New York City in 1971, when he was 13 years old. In 1979, he took up Taekwondo and Karate, earning black belts. He graduated in 1975 from the Admiral Farragut Academy in Pine Beach, New Jersey.
Longing to be an actor since the age of five, Lamas first studied acting in Tony Barr's Film Actors Workshop and quickly thereafter obtained his first TV acting role in 1976. As a last-minute replacement for Steven Ford, Lamas secured a non-speaking role as a jock in the 1978 film Grease in which he dyed his hair blond. Early in his career, he also had guest-starring roles in Switch, Sword of Justice, Dear Detective, Secrets of Midland Heights, Fantasy Island, The Love Boat and Hotel.
In 1980, Lamas auditioned for and won the role of Lance Cumson, for the pilot of a new series entitled The Vintage Years. The pilot was later retooled to become the hit prime time drama series Falcon Crest. During a 2006 TV interview with a Norwegian television team, Lamas said that to get the role on Falcon Crest, he had auditioned twice and beat out five other guys for the part. During his stint on Falcon Crest, Lamas was nominated for two Soap Opera Digest Awards and a Golden Globe Award. He was the only actor to appear in all 227 episodes of the series. In an interview, Lamas said that Jane Wyman's persona was more of a hard-working legend, not a diva, who referred to her as a friendly woman on Falcon Crest: "With Jane, she expected you to know your lines, but she played poker with the crew at lunch. She was just a great gal; and I think that set the tone for the rest of us. She wasn't a 'spoiled diva,' and believe me, if anybody had a right to be a 'spoiled diva,' it was Jane because she has done so much, Oscar Award-winner, I mean, incredible actress, and she was just like clockwork, right there on time, always knew her lines, always ready to rehearse and she had this great affable quality. You just never talked about (Wyman's ex-husband and the then-current U.S. President) Ronald Reagan, that's all. That was one thing we never did; everything else was opened." At the beginning of the ninth and final season of Falcon Crest, off-camera, Lamas received some devastating news about his TV grandmother when she was sent to the hospital that he paid visit to see her to offer her some words of comfort. After cancellation, he kept in touch with her for years; and on September 10, 2007, he lost his TV grandmother and decades-long friend. Prior to Wyman's death, Lamas released a statement: "Next to my parents, Jane was the most influential person in my young career," he added, "She has left an incredible body of work and accomplishments that cannot go without being recognized and celebrated. I will miss her greatly."
In 1984, Lamas was nominated for Worst Actor at the Golden Raspberry Awards for his performance in the film Body Rock. He also performed a song on the soundtrack for this film, and the track "Fools Like Me" became his only single to date to crack the Billboard Hot 100 chart. In the early 1990s, he began carving out a niche for himself as an action-hero, showcasing his martial-arts skills by starring in such movies as the Snake Eater-trilogy, Terminal Justice, Gladiator Cop, Bounty Tracker, and many similar low-budget action-films. In 1992, Lamas played the role of the falsely accused cop Reno Raines in the syndicated series Renegade. The series was seen in over 100 countries, and during its final season, it moved from first-run syndication to the USA Network. The show ended in 1997 after a run of five seasons. From February 2004 until February 2007, he played the role of Hector Ramirez on the CBS daytime soap opera The Bold and the Beautiful.
In August 2007, Lamas starred as the King of Siam in The King and I at the Ogunquit Playhouse in Ogunquit, Maine. That fall, he performed at Kean University Premiere Stages in Union, New Jersey, in the title role in Steven Dietz's Dracula. He performed as El Gallo in The Fantasticks at the Casa Mañana Theatre in Fort Worth, Texas, in June 2008. In June 2009, Lamas returned to the Ogunquit Playhouse as Zach in A Chorus Line.