On October 4, 1957, Mestral was aboard a Vulcan survey ship documenting the launch of Earth's first artificial satellite, Sputnik 1. Shortly after, the ship crash-landed near the town Carbon Creek, Pennsylvania, in the United States of America. Mestral was one of the three survivors, along with T'Mir and Stron.
The three Vulcans acquired local attire to disguise themselves during their interactions with the resident species, the humanoid Humans. Mestral's understanding of geometry allowed him to acquire dollars, the local currency, in a table game.
In the following weeks, the trio settled in a house in town and acquired jobs. Mestral and the owner of the local tavern, Maggie, became attracted to one another, but Mestral kept his true nature a secret.
Eventually, a Tellarite freighter received the Vulcans' distress signal and passed it on to the Vulcan High Command, who dispatched the D'Vahl to retrieve the survivors. Deeply fascinated by the Humans, Mestral asked his colleagues to pretend he died in the original crash. T'Mir and Stron agreed, and the D'Vahl departed without Mestral. (ENT episode: "Carbon Creek")
By January 22, 1958, Mestral had filled notebooks and his scanner with information about Human cultures, and was packing to leave Carbon Creek the day after. Maggie had been interested in becoming intimate with Mestral. While Mestral was packing, he was attacked by the Certoss Jaecz. After subduing the Certoss, Mestral engaged in a mind-meld with the attacker and learned that Jaecz was a temporal agent from the future. The Certoss and his three fellow time travelers were on a mission to destroy Humanity. Mestral spares Jaecz's and left Carbon Creek right away.
Knowing he would be unable to stop the Certoss agents on his own, Mestral visited Wright-Patterson Air Force Base in Dayton, Ohio, on July 29. The installation operated by the US-American government was the operations center of Project Blue Book, headed by Major James Wainwright and Staff Sergeant Allison Marshall.
The trio went to Mestral's apartment, where Major Wainwright consented to a mind-meld. Mestral unearthed suppressed memories of Wainwright and Allison finding the ship's wreck in Carbon Creek. Unbeknownst to them, agents of the Aegis had suppressed their memories and removed evidence of the ship's existence. Mestral considered Earth his home now and sought to protect his friends from harm. Wainwright and Marshall introduced Mestral to their superior, Professor Jeffrey Carlson of Majestic 12. Nobody else in the US government was informed about Mestral.
On September 26, 1963, the Certoss agents Adlar and Gejalik attempted to access a Vulcan probe stored at Wright-Patterson. Gejalik considered it possible that it had been sent to collect Mestral. Jaecz believed Mestral dead by this point.
One day later, Mestral took part in a meeting between Wainwright, Marshall and Professor Carlson, informing them that the probe was intended to collect data and transmitting it to the Vulcan Science Academy. The three Humans were relieved by the Vulcans' peaceful intentions and figured out the Certoss may have been interested in the probe's long-range communications equipment.
The defenders of Earth moved to apprehend the Certoss agents in 1968. Unbeknownst to them, they were supported by a pair of Aegis operatives. Mestral had spent months studying the personnel at NASA facilities in Florida and Texas to identify the Certoss agents masquerading as Humans, and had developed a crude scanning device based on a Geiger counter.
Mestral traveled to New York City on April 3, where he found Gejalik in the apartment of the Aegis agents. She attempted to access the Beta 5 computer, which had been hacked by Jaecz from Trenton, New Jersey. During the standoff between Mestral and Gejalik, both of them were transported to the year 2268 when the Aegis translocator was triggered.
Prior to World War III, Lily Sloane had befriended an elderly Mestral. When the war began in 2053, Mestral and a young scientist he met, Zefram Cochrane, traveled the distance from Boston, Massachusetts to Bozeman, Montana to partner with Sloane and build Cochrane's prototype warp engine. (ENT - Strange New Worlds 9 short story: "Mestral")
In 2067 - about four years after official contact was made between Vulcans and humanity - Mestral and Gary Seven witnessed the launch of Friendship One from the control center in New York City. (TNG novel: Hearts and Minds)
While in pursuit of the Certoss operative Gejalik, both of them ended up on the USS Enterprise in the year 2268. Mestral filled Captain James T. Kirk and Commander Spock, a fellow Vulcan, in on his experiences. When a Tandaran starship engaged in battle with the Enterprise trying to claim the time-travelers, Aegis operative Roberta Lincoln brought Mestral and Gejalik aboard the Certoss vessel UEV Balatir, which Kirk had secured in the hangar bay to prevent its capture by the Tandarans.
Eventually, Gejalik offered to turn herself over to Colonel Abrenn to keep her world safe, and Mestral suggested to come along to corroborate her story. Fortunately, Kirk convinced Abrenn to admit defeat. Kirk and Spock assisted Lincoln and capturing the remaining Certoss operative, Adlar. Both Adlar and Gejalik returned to Certoss Ajahlan, while Mestral accompanied Lincoln back to the 20th century to continue his studies of Humanity. (TOS novel: From History's Shadow)