Mary Jane is depicted as an extremely beautiful, green-eyed redhead, and has been the primary romantic interest of Peter Parker for the last twenty years, although initially competing with others for his affection, most prominently with Gwen Stacy and the Black Cat. Mary Jane's relatively unknown early life was eventually explored in Amazing Spider-Man #259.
Early issues of Amazing Spider-Man featured a running joke about Peter dodging his Aunt May's attempts to set him up with "that nice Watson girl next door", whom Peter had not yet met and assumed would not be his type, since his aunt liked her (in the Parallel Lives graphic novel an identical scenario is shown between Mary Jane and her Aunt Anna). Mary Jane made her first actual appearance in Amazing Spider-Man (Vol. 1) #25 (June 1965); however, in that issue, her face was obscured. It is not until Amazing Spider-Man (Vol. 1) #42 (November 1966) that her face is actually seen. In that issue, on the last page, Peter finally meets her, and he is stunned by her beauty even as she speaks the now-famous line: "Face it, Tiger... you just hit the jackpot!" Mary Jane's face is shown for the first time. Art by John Romita Sr. From Amazing Spider-Man #42.Peter begins to date her, much to the annoyance of Gwen Stacy. However, her apparent superficiality proves to be an irritation to Peter that her rival did not share. Peter eventually learns to cope with this, and Mary Jane becomes an occasional flirtatious interest as well as a close friend to Peter, Gwen, and others.
Despite her enjoyment of life, her friendships, and dating, Mary Jane refuses to be tied down for too long. When her relationship with Harry Osborn comes to an end, it has significant impact on Harry, driving him to a drug overdose. This in turn creates a boomerang effect, driving his father Norman Osborn to the brink of insanity, temporarily restoring his memories as the Green Goblin. Mary Jane only realizes the true consequences of her lifestyle when she learns of Harry's predicament.
Later, when the Green Goblin murders Gwen, MJ stays with Peter during his mourning; though he initially tells her to leave him alone, he becomes interested in her as he recovers. Their relationship has a few initial hurdles, such as MJ's hot temper and Peter's always dashing off to be Spider-Man. Following the events of the original clone saga, Peter realizes that Mary Jane is the girl he has always loved, and the two begin dating again.
However, despite loving Peter, MJ does not wish to be tied, and when she allows the relationship to progress too far, she is left with a difficult decision when Peter proposes to her. After taking a short time to consider, she turns him down. Following a series of traumatic experiences involving Peter's absences and his costumed alter ego endangering his Aunt May, a spiritually-exhausted MJ leaves New York for several months. Peter meanwhile dates other women, most notably Felicia Hardy.
MJ eventually returns, her behavior showing a marked change with her abandonment of her false front. Following an attack on Peter by Puma, she breaks down and admits her knowledge of Peter's secret identity in Amazing Spider-Man (Vol. 1) #257. After learning of her own family history in Amazing Spider-Man #259, Peter finds a new respect for her and begins to truly understand her. MJ however, makes it clear to Peter that knowing his identity changes nothing about her feelings, and that she only loves him as a friend.
Despite the one-shot graphic novel "Parallel Lives" and Untold Tales of Spider-Man #16 revealing that Mary Jane discovered Peter's secret when she noticed Spider-Man climbing out of Peter's bedroom window, many comics published before this revelation claimed that she had simply "figured it out", with the details of how and when left ambiguous to the reader.
After yet another period of reconsidering his priorities in life, Peter contemplates letting go of the Spider-Man mantle, with Mary Jane backing the decision, but his relationship with Felicia Hardy soon resumes. Feeling lost and guilty, Peter visits Mary Jane and apologizes with an awkward kiss before heading to Berlin with Ned Leeds.
Following Ned Leed's murder at the hands of the Foreigner, a changed and bitter Peter returns to New York, where his lack of direction in life is not helped when Ned is framed as the Hobgoblin, and Felicia elects to leave Peter behind as she is tied to the Foreigner. Mary Jane returns to Peter, presumably to patch things up, but Peter surprises her with a second proposal of marriage, which MJ again turns down. She returns to her family to settle old debts with her father, with Peter following her. After aiding her sister in having her crooked father arrested, and aiding Peter against a Spider-Slayer, Mary Jane has an epiphany on marriage, and agrees to become Peter's wife.