|First seen||'Return of the Kane'|
|Last seen||'There's Got to be a Morning After Pill'|
|Occupation||Neptune High student|
|Family members||Mrs Sinclair (mother)|
Mr Sinclair (father)
Lauren Sinclair (sister)
Natalie Mackenzie (biological mother)
Sam Mackenzie (biological father)
Ryan Mackenzie (biological brother)
Sheriff Don Lamb
|Portrayed by||Amanda Noret|
Madison Sinclair is the "resident bitch" of Neptune High. She epitomizes the ditzy cheerleader: she is shallow and spoiled, and dated Dick Casablancas, one of Neptune High's most arrogant and self-centered young men. She also single-handedly rigged a school election in order to get Duncan Kane elected to student body president. Veronica exposed the plot which resulted in Madison losing her place on the student council and the special privileges granted by the position.
Madison likes to spit in the drinks of people she doesn't like, which she calls "A Trip to the Dentist." When she spit into a cup of spiked drink meant for her and gave it to Veronica, she set off a chain of events that led to Veronica being raped.
Despite not being with Dick anymore, she still had feelings for him at the beginning of Season Two. While at a slumber party hosted by Gia Goodman, she learned that Gia was now dating Dick and was visibly upset. However, by the end of the season, she appears to be over him as she is involved secretly with Sheriff Don Lamb (revealed in 2x16 "The Rapes of Graff").
Veronica had identified Madison to her father when Keith Mars discovered Lamb was dating a mysterious blonde woman. She then ruthlessly mocked Madison about her relationship with Lamb in front of their classmates through innuendo that, while her classmates didn't understand, had a meaning that was quite apparent to Madison, who was visibly shaken as a result.
Madison, having gone off to college at USC, is absent from much of Veronica's life at Hearst, until she had several run-ins with her in Neptune in the episode "Poughkeepsie, Tramps and Thieves". Veronica first sees Madison when she arrived unexpectedly at Neptune Grand Suite shared by Logan and Dick; thinking that Madison was simply looking up her ex, Veronica sent her on her way.
Later, however, she ran into her again at a lingerie shop where Madison maliciously let slip that she had spent the night with Logan while in Aspen over Christmas break, which is when Veronica and Logan were split up.
In the following episode, Veronica, consumed by images of Madison's tryst with Logan, follows her and eventually plots with Weevil to have Madison's new car crushed and cubed. Veronica has a change of heart, however, and asks him to return it unscathed (save for a can of tuna in her air conditioning).
Madison was switched at birth with Mac, one of Veronica's friends. As such, the self-absorbed Madison is often at odds with her more cerebral parents and younger sister, though her parents have gone out of their way to spoil Madison rotten. However, in spite of all of this, Madison's "mother" does harbor some resentment towards her daughter and longing to know her true biological daughter Mac. Ironically though, Mac's parents have fully accepted Mac as their daughter and have shown no signs of wanting anything to do with Madison.
- Veronica Mars