"That's why you're staying aboard."
At some point in his Starfleet career, Decker held a posting on the planet Delta IV, where he entered into a romantic relationship with the Deltan Ilia. Decker ultimately chose to end the relationship, however, subsequently leaving Delta IV without saying goodbye to Ilia.
Decker was recommended by Kirk to command the refitted USS Enterprise following the completion of Kirk's legendary five-year mission and subsequent promotion to rear admiral. Despite his recommendation, Kirk conveyed his feelings of envy towards Decker, telling the younger officer his hopes of commanding a starship again in the future. The now-captain Decker went on to oversee the extensive refit of the Enterprise throughout its entire eighteen months.
In the 2270s, when it became known that an extremely powerful force was en route to Earth, Kirk seized upon the opportunity to convince Admiral Nogura to grant him command of the Enterprise in order to intercept the threat. Kirk gave Decker a temporary grade reduction from captain to commander, and reassigned him to executive officer until the intruder was dealt with, something Decker protested. Decker bluntly expressed his belief that Kirk was only using the emergency to retake command of the Enterprise, and that his inexperience with the newly-upgraded vessel could jeopardize the mission. Regardless, Kirk remained adamant that his own experience in dealing with unknown threats was pivotal to their mission, and maintained command. Later, after the unexpected death of Commander Sonak in a transporter accident, Decker was also forced to double as science officer. This assignment proved only temporary, however, when Spock arrived to offer his services.
Prior to the Enterprise's launch, Decker received the surprising but pleasant news that his former love, Ilia, now a lieutenant, had arrived aboard the Enterprise to serve as navigator. When they were alone, Decker apologized to Ilia for leaving her, but Ilia made it clear that she was still harboring negative feelings regarding the unresolved manner in which their relationship ended. They had little time to rekindle their relationship, however, as Ilia was taken from the bridge by a probe from the enemy vessel, reduced to a data stream, and replaced by a probe that duplicated the Deltan officer's form.
When it was found that the probe not only copied Ilia's appearance but her memories as well, Decker was given the assignment of reviving Ilia's memories, in an attempt to make contact with her and learn more about the entity approaching Earth, whom the crew had learned called itself "V'ger." His efforts proved to be a partial success, as, for a moment, Ilia's personality was able to surface and explain to Decker that V'ger was seeking an unknown "creator" on Earth.
Upon arriving at Earth, V'ger threatened to destroy the planet unless it was allowed to meet with the creator. Decker was among the landing party that also included Kirk, Spock, and Doctor McCoy, which discovered that V'ger was actually an old Earth space probe known as Voyager 6 and was seeking to evolve by joining with its creator, namely, a Human. Having lost the captain's seat and no longer having Ilia, Decker volunteered to physically join with V'ger. Decker and the Ilia probe disappeared into a blinding source of light, evolving into a new lifeform, leaving for parts unknown.
Upon returning to the Enterprise, Kirk issued orders that both Decker and Ilia be officially listed as "missing." (Star Trek: The Motion Picture)
"Admiral Kirk! Well, we're getting a top brass send off. Don't worry, we'll launch on schedule, even if we have to tow her out with our bare hands, right Scotty?"
- - Will Decker, to Kirk when he arrives on the Enterprise (Star Trek: The Motion Picture)
"I'm sorry, Will."
"No, admiral. I don't think you're sorry. Not one damn bit. I remember when you recommended me for this command. You told me how envious you were, and how much you hoped you'd find a way to get a starship command again. Well sir, it looks like you found a way."
"Report to the bridge, commander. Immediately."
- - James T. Kirk and Will Decker, on Kirk taking command of the Enterprise (Star Trek: The Motion Picture)
"Permission to speak freely, sir?"
"Sir, you haven't logged a single star hour in two and a half years. That, plus your unfamiliarity with the ship's redesign, in my opinion, sir – seriously jeopardizes this mission."
- - Will Decker, confronting James T. Kirk (Star Trek: The Motion Picture)
"Captain, as your exec, it's my duty to point out alternatives."
- - Will Decker to Kirk, on his duty as executive officer (Star Trek: The Motion Picture)
"Moving into that cloud, at this time, is an unwarranted gamble."
"How do you define unwarranted?"
"You asked my opinion, sir."
- - Will Decker and James T. Kirk, on whether or not to go into V'ger (Star Trek: The Motion Picture)
"This is how I define unwarranted!"
- - Will Decker to James Kirk, after V'ger vaporizes Ilia (Star Trek: The Motion Picture)
"You mean this machine wants to physically join with a Human? Is that possible?"
"Let's find out."
- - Leonard McCoy and Will Decker (Star Trek: The Motion Picture)
"Jim, I want this! As much as you wanted the Enterprise, I want this!"
- - Will Decker, before joining up with V'ger (Star Trek: The Motion Picture)
Will Decker was played by actor Stephen Collins.
Commander Will Decker was to have featured as the Enterprise's first officer in Star Trek: Phase II. When this project fell through due to scheduling conflicts between actors and directors, Will Decker was used in the movie which utilized the script of the unmade pilot episode of Phase II "In Thy Image."
Stephen Collins felt that the easiest aspect of being Decker was landing the role. He recalled, "It seemed pretty simple to me at the time. They had been looking for someone for a while. I was called in for a meeting with Gene while I was in Los Angeles working on a TV show. He thought I fit the overall idea of what they were looking for – a young Kirk in a way, but someone who could still command authority. Several days later, I went to audition for Robert Wise, read a couple of scenes and that was it! They offered me the part the next day." (Starlog #104, March 1986, p. 60)
Collins felt that an important part of why he was cast in the role of Will Decker was that, unlike many or all of the other candidates for the part, he did not revere Star Trek. "I think it really helped me get the part," he said, "because I walked into the meeting and, you know, it was just a part to me. I mean, I was much more interested in meeting Robert Wise than I was at the thought of doing Star Trek." Collins auditioned for Wise and, by the time he returned home, not only had Wise decided that Collins was the best applicant for the role but an offer had also been struck up for him to portray the character. ("A Bold New Enterprise - The Making of Star Trek: The Motion Picture", Star Trek: The Motion Picture (The Director's Edition) DVD special features)
While playing Will Decker, Stephen Collins often felt as if he was somewhat intruding on the bubble-like atmosphere of the main cast, virtually all of whom had been in Star Trek: The Original Series (with the exception of Persis Khambatta as Ilia). "Some of that really helped in terms of the part I played," Collins mused, "because I was supposed to be this outsider who they had to have along on the trip." ("A Bold New Enterprise - The Making of Star Trek: The Motion Picture", Star Trek: The Motion Picture (The Director's Edition) DVD special features)
Collins had a particularly bad memory of how the script was written for his role as the "humorless" Decker, "It was very difficult for me. When I was offered the part of Decker, about two-thirds of the script had been written and that last third was in scenario form. In that last part, Decker really emerged as the story's center because he was being groomed to go off with Ilia. But when the film got made, Decker's importance was minimized. What I was offered never came to fruition. At the end, I did merge with V'ger and Ilia, but it didn't happen that way in the scenario and it was very disappointing to me." Collins, in part, blamed this on the fact that William Shatner and Leonard Nimoy were given the ability of script approval, thus taking the focus off Decker and weakening his role. He ended by stating his opinion that "Decker was not very well-rounded and became sort of two-dimensional and uninteresting. [....] Decker is too stolid and lockjawed, so there wasn't much room for humor or real human behavior. As a result, many people think of me that way." (Starlog #104, March 1986, p. 59)
In 1978, Andrew Robinson and Stephen Macht were among those who auditioned for the role of Commander Will Decker in Star Trek: The Motion Picture. (The Making of Star Trek: The Motion Picture, p. 104)
As Matt Decker's son
Early indications of Decker's heritage appeared in the reference guide Star Trek Phase II: The Lost Series, which stated that Will Decker descends from numerous other Starfleet officers, some of whom are of flag rank.
The question of a potential relationship between the two Deckers was addressed in the "Star Trek Report" by Susan Sackett in the July 1979 issue of Starlog (#24, p. 31), which featured a few fan questions. Sackett's printed response was "There is no mention of Commander Decker's parentage anywhere in our script; however, Gene did have this in mind when he created the character, and I believe you will see certain father-son similarities of character and integrity." She repeated this information as a footnote in the reference book, The Making of Star Trek: The Motion Picture, where she identifies Will Decker's father as "Commodore Matt Decker of the Star Trek television episode 'The Doomsday Machine'." It was again reiterated in the April 1988 issue of Starlog (#129, p. 24).
StarTrek.com, the official Star Trek website, also supports this idea, as it clearly states that Captain Decker is the son of Commodore Matt Decker.  For the entry about Will Decker in the Star Trek Encyclopedia (4th ed., vol. 1, p. 191), he is identified as the son of Matt Decker. In a background note for this entry, the authors state that Paramount publicity materials for the first movie suggested the existence of this relationship.
The backstory regarding Will Decker's relationship with Lt. Ilia was later "borrowed" for the characters of William T. Riker and Deanna Troi when Star Trek: The Next Generation went into production. Riker duplicated many of Decker's character traits, as well, at least in TNG Season 1.
The article "Star Trek Discovery's Coolest, Deepest-Cut Easter Eggs" by StarTrek.com suggests that it is likely that a Cadet Decker referred to in DIS: "Into the Forest I Go" may actually be this Decker, in his youth. 
According to the script and most other sources, Decker's full name is Willard Decker. However, the Star Trek: The Next Generation Officer's Manual calls him William Decker.
It was clearly established in the novelization of Star Trek: The Motion Picture that Will Decker was the son of Commodore Matt Decker. The novel Ex Machina gives his mother's name as Joan Decker, while Who's Who in Star Trek 1 calls her Jane Decker. The video game Star Trek: Starship Creator calls her Amy Decker.
In The Sorrows of Empire, his mirror universe counterpart was depicted as being the first officer of the ISS Enterprise from 2270 to 2277. After a failed attempt to assassinate Spock on Deneva at the behest of his father Grand Admiral Decker, he was killed using the Tantalus field. Grand Admiral Decker swore vengeance against Spock but he was killed himself shortly afterwards.
The story "Consider Eternity" in the Waypoint Special reveals that shortly after his joining with V'ger and the Ilia probe, Q invited them to a meeting wherein he temporarily separated Decker from the rest of their combined being to allow Decker the time and option to consider his real motivations for joining with the Ilia probe and V'ger. Although Decker began showing second thoughts, he ultimately decided to rejoin the three and subsequently thanked Q for his guidance. Decker's meeting with Q then became a fond memory for Q that inspired his intervention with Captain Jean-Luc Picard and the crew of the USS Enterprise-D as they headed off to Farpoint Station.