At age 14, he left PMA and returned home to attend George Washington High School. He was greeted at home by a new addition to the family; a baby sister, Valerie. On graduating in 1942, he drifted from one job to another until he caught the eye of a professional photographer. The woman immediately recognized a man who fit the classic stereotype of tall, dark and handsome and she suggested he try modeling. To Armando, the profession sounded like easy money and he decided to give it a try, much to the chagrin of his father who had hoped Armando would join him in the insurance business. About this same time, Armando also became interested in acting. An agent suggested a name change to help the aspiring actor avoid being type cast in ethnic roles. Armando Catalano became actor and model, Guy Williams.

In 1947, Guy landed his first acting assignment, a small role as a bombardier in the WWII movie The Beginning or the End, a film about the first atomic weapon and the bombing of Hiroshima. His bit part in the movie only increased his desire to act full-time, but roles were hard to come by and since he preferred eating to starvation, he accepted modeling assignments to support himself. It was during a modeling assignment photo shoot for a Mum's Deodorant ad that Guy met Powers model, Janice Cooper, a native of Bolivar, Tennessee. According to both Guy and Janice, it was a case of love at first sight. After a whirlwind courtship, the couple married December 4, 1948. The newlyweds each continued their modeling careers while Guy also honed his acting skills by studying under Sanford Meisner at the Neighborhood Playhouse.

By the end of the 40s, Guy was signed by MGM and went to Hollywood to make his first movie. Subsequent roles were scarce and he returned to New York, where Janice was expecting. The year 1951 brought a note of sadness to Guy's life. His father passed away, without ever seeing his son succeed in the acting career he'd chosen. The Williams' first child, Steve, was born in 1952. To support his growing family, Guy again took up modeling. A short time later, he was signed to a contract by Universal Pictures and this time, he took his family with him when he moved to Hollywood. While under contract to Universal, he appeared in a series of movies in supporting roles. In an interview, Guy assessed his early career by saying "I used to play anonymous men leaning in doorways with cigarettes dangling from their lips. There were times when I seriously doubted if I were cut out for this business."

In 1954 a riding accident on the studio lot ended this phase of his budding acting career. According to Janice, "Guy was riding bareback on the back lot at Universal Studios where he was under contract. Something scared the horse. He stopped and Guy kept going. Guy's shoulder was completely dislocated and broken...and he was in a half body cast for some time." According to other sources, Guy and another actor were involved in a friendly race when the accident happened. Surgery was required on his shoulder to repair the damage and as a permanent souvenir of the incident, he bore a long and prominent scar on his shoulder. In a 1958 interview, Guy commented about his shoulder injury, saying "it only hurts today when I think about it."

When his contract with the studio expired, he was released due to his inability to work. Finding himself unemployed, as well as needing to rehabilitate the impaired muscular control of his injured shoulder, Guy decided to take up the sport of fencing again, this time under the tutelage of world-famous fencing coach Aldo Nadi. It was a decision that paid off for the actor. Within six months he had regained full use of his left arm and become an expert fencer in the process. In an interview, Guy gave his thoughts on his sport, "Fencing is a marvellous sport. It keeps you in good physical trim and it's exciting." His choice of rehabilition not only improved his health, it revitalized his career. In 1957, Guy was in Hollywood to make the movie, I Was a Teenage Werewolf. And for once, good fortune shined on him. His agent informed him that Walt Disney was searching for an actor to play the lead in a new television series, Zorro.

Television, which was in its infancy at the time, was generally disparaged by actors, but Guy saw it as an opportunity. He was later quoted as saying that Walt Disney was known for the quality of his productions and the chance to work for such a respected studio made the idea of acting in a television series attractive. On April 18, 1957, Guy tested for the lead role in Zorro and was selected over some of Hollywood's more well known actors. The show was a hit and the role catapulted Guy to overnight stardom. In an interview, he gave his opinion on his profession, saying, "getting paid for having fun is one of the best occupations I know of!"

While he enjoyed the young fans who flocked to him in droves during his publicity appearances that among other places included Disneyland, Guy wasn't enchanted by the idea of celebrity. An aficionado of classical music, Guy preferred hobbies to the usual Hollywood party scene. Among his favorite pastimes were sailing, astronomy, gourmet cooking, raising tropical fish, photography and the cerebral game of chess, at which he was considered an expert.

In 1957, at the height of his popularity, Guy became a father again when Janice gave birth to daughter, Toni. It was also during this time that rumors of trouble in their marriage began to surface. At least one magazine reported that the couple had separated. Whether the rumors were true or not, they remained together.

When Zorro was cancelled in 1959, Guy and his family toured Europe for several years while he filmed a series of movies; The Prince and the Pauper, Damon and Pythias, and Captain Sindbad. The chance to join the cast of the hit series Bonanza in 1964, enticed him back to Hollywood. While he only appeared in five episodes of the western, it was enough to make at least one producer take notice. Irwin Allen was casting parts for his new Sci-Fi television series, Lost in Space and signed Guy to play the lead in the show.

The 60s were turbulent times for the United States and Guy had what at the time were decidedly controversial opinions about many issues in the news. Using a term popular now, he was not politically correct. He was an outspoken opponent of the U.S. involvement in Vietnam and was one of the first Hollywood stars to speak out against the war. Angered by the political games being played by both Republicans and Democrats, Guy at one time considered running for office. He was quoted in an interview as saying he would "cut the baloney" out of government. He was a strong proponent of the civil rights movement, was pro-choice on the issue of abortion, thought marijuana and gambling should both be legalized and disavowed any form of censorship. Whether his controversial opinions damaged his acting career is a matter of speculation.

During the three year run of Lost in Space, more rumors of marital strife surfaced. The couple came from vastly different backgrounds, with Guy being a stereotypical Sicilian New Yorker and Janice a more reserved Southern lady. Doubtlessly, their different personalities strained the marriage as much as Guy's stardom and the inevitable crush of adoring female fans and admirers he attracted.To make matters worse for him, the role of John Robinson in Lost in Space was not the vehicle for stretching his acting ability that Guy hoped for or was promised by Irwin Allen. And long before the series ended after the 1969 season, Guy had become disgusted with the direction of the series.

In the mid-70s, Isabel Peron enticed Guy to come to Argentina to make a publicity appearance. Thoroughly disillusioned with Hollywood and the lack of good roles coming his way, Guy was amazed by the reception he received in Argentina. After several years of jaunting back and forth between LA and Buenos Aires, he eventually moved to the Argentine capital. The move was the beginning of the end for his marriage and in 1984, Guy & Janice divorced. Guy remained in Buenos Aires, where he died in May 1989.