Jarvis served in the British RAF and was their undefeated boxing champion three years running. After the war he moved to America where he became butler to Howard and Maria Stark and watched over their mansion even after their deaths.
The Starks' son, Tony, called the first meeting of the Avengers as Iron Man and donated the house as headquarters, so that it became Avengers Mansion. Jarvis grew accustomed to the guests and served the Avengers for many years thereafter, acting as a father figure to some of the newcomers. He was the only one to stay with the Avengers for their entire existence, a distinction not even Captain America can claim. As a result, Cap has stated that Jarvis should be regarded as just as much of an Avenger as he is.
Jarvis spent some time as the primary babysitter for Franklin Richards, the super-powered son of Mr. Fantastic and the Invisible Woman when they were residing at the mansion. He also served as the sponsor to future Avengers member Silverclaw while she was growing up, and she has come to regard him as an uncle.
Being butler to the Avengers meant that Jarvis has had to deal with their enemies himself on several occasions. In Avengers #59 and #60 (December 1969, January 1970) he was assaulted by intruders into the mansion and bound and gagged. He has also been involved personally in many adventures, including leading the evacuation of a stalled subway train during a city-wide disaster and battling a demonically possessed car. During this time he dated a woman he rescued named Glory. Jarvis defended the floating Hydrobase against a horde of robots built by Doctor Doom during the Acts of Vengeance crisis. He was present when one of the many incarnations of the Masters of Evil attacked the mansion. He was brutally beaten while Captain America was forced to watch. It took some time for Jarvis to recover from his injuries. He even wore an eyepatch for some time. Jarvis personally confronted Loki, risking great danger, after realizing the villain had tricked his way inside the mansion in Avengers #400.
In his duties as the Avengers' butler, he was entrusted with items of great power, including the Casket of Ancient Winters.
When the New Avengers were formed, Jarvis was called back after taking a holiday "for the first time in years," having been informed that his 'special' services were once again needed. Jarvis often confronts Wolverine over the man's poor kitchen etiquette, a battle only new member Spider-Man's Aunt May was able to win. Jarvis struck up a relationship with May, who had moved into Stark Tower with Peter and Mary Jane after her house burnt down. When Spider-Man switched sides during the Civil War, however, Aunt May and Mary Jane fled Stark Tower to live in hiding.
In a recent New Avengers "Civil War" story, Jarvis was shot by an employee of Tony Stark, who was opposed to Stark using technology he invented to enforce the Superhuman Registration Act. However, it appears Jarvis recovers from this wound, as in Civil War: The Initiative, he is shown to be working again. He also humorously mentions that if Tony Stark allowed "that Tigra %^#$" in the new incarnation of the Avengers, Tony would need to find someone else to do the laundry. Tigra had worked for Tony Stark's side throughout the entirety of the Civil War incident.
In Spider-Man: One More Day, Jarvis is given a slightly over $2,000,000 check by Stark to pay for Aunt May's hospital bills after an angry confrontation with Peter. She was dying after being shot by a sniper (hired by The Kingpin) in revenge once Stark convinced Spider-Man to unmask. Jarvis visibly breaks down upon seeing her in the hospital bed, confessing his deep love to the Parkers.
As a consequence of the "One More Day" storyline, the Spider-Man comics have undergone a major continuity overhaul, of which many consequences have yet to be revealed. With Peter Parker's identity secret again, there may be no way to connect May and Jarvis; an issue of New Avengers is advertised as addressing Spider-Man's "new" history with the team, which would include this time period.
In Secret Invasion, it is revealed that Jarvis has been replaced by a Skrull agent (though the exact time is unclear, after the events of Secret Invasion, it was revealed in a summary of Secret Invasion, when describing the Jarvis Skrull, that it was apparently before Jarvis started his relationship with May). Using a computer virus, he disables a number of Stark Enterprises facilities, as well as Stark's Iron Man armor. This agent also obtained access to Tony Stark's records on Robert Reynolds (The Sentry) in order to discover his weaknesses, as the Skrulls were unable to duplicate his powers because its limits were unknown even to Stark. He later approaches Maria Hill on the destroyed S.H.I.E.L.D. Helicarrier in the middle of the ocean and tells her to surrender along with her crew. She uses a Life Model Decoy in that spot as the real one escaped and detonated the S.H.I.E.L.D. Helicarrier. During the fight between Veranke and Criti Noll's forces against the heroes and villains, The Skrull-Jarvis (who somehow survived the S.H.I.E.L.D. Helicarrier explosion) watches from Avengers Tower as the energy washes over hero, villain and Skrull alike. While holding Jessica Jones and Luke Cage's child, he comments that it doesn't matter if the Skrulls win or lose, so long as "God's" will is done.
After the final battle, the real Jarvis is discovered alive, prompting Jessica Jones to discover that her baby had been taken by the Skrull impostor. Jarvis is later shown in a support group meeting with the others that had been replaced by Skrulls. The Skrull Jarvis was eventually found in a hideout and was shot by Bullseye after returning the baby to Luke Cage.
During the "Earth's Mightiest" arc, the real Jarvis, having refused to serve under Norman Osborn's new group of Avengers is sought out by Hercules and Amadeus Cho to be a part of the new Mighty Avengers team, Amadeus having done the math and found Jarvis to be the one constant of the Avengers