Messick was the daughter of Michael Sobel and Actress Laura Nicholson Sobel (1945–2015). She was the maternal granddaughter of film Producer James H. Nicholson and had a brother named Jan. After attending Santa Barbara High School, where she was named homecoming Princess, Messick graduated with a degree in communications from the University of Southern California.

After being criticized for initially not commenting on Messick's death, McGowan paid tribute to Messick on Instagram without addressing her family's accusations. Messick was among the deceased memorialized at the 90th Academy Awards.

She began her career at the Gersh Agency and later worked as a junior manager at Somers/Teitelbaum/David. She was Director of development at Woods Entertainment from 1994 to 1996, working on films including Scream (1996) and Teaching Mrs. Tingle (1999), both distributed by Dimension Films, a division of Miramax Films. She also helped facilitate Miramax distributing M. Night Shyamalan's Wide Awake (1998). Shyamalan later said that "she helped fight for my second film to get made at Miramax. Really made me feel I had a big sister protecting me." In the late 1990s, she worked as a talent manager at Addis-Wechsler & Associates, where her clients included Chloe Sevigny and John Bloom. In November 1997 she was hired as vice-president of development by Miramax Films and tasked with "scouting Writers and Directors and bringing in material to the company." She reported directly to executive vice-president of production Meryl Poster and was hired by Cathy Konrad, who said that she had "a keen eye for spotting new talent."

She was Actress Rose McGowan's manager in 1997 when McGowan alleges that she was raped by Harvey Weinstein. After the Weinstein allegations became public in late 2017, McGowan was highly critical of Messick; and Weinstein's Lawyer released a private email sent by Messick that contradicted McGowan's claims. The publicity surrounding the scandal led to online cyberbullying directed at her. Messick committed suicide in February 2018. In a public statement, Messick's family said that she supported the Me Too movement and called her loss collateral damage of a "feeding frenzy", blaming Weinstein, McGowan, the media, and the public for her death.

At Miramax Messick helped bring She's All That (1999) to the company within a month of her hiring, It eventually grossed $60 million. She worked on such films as Boys and Girls (2000), Frida (2002) and Cinderella Man (2005). She also worked directly with Dimension Films and helped make multipicture deals with screenwriters R. Lee Fleming, Jr., Andrew Lowery and Andrew Miller. In 2000, she was promoted to senior Vice-President of Production. She worked at Miramax until 2003. She later worked as an executive Producer for Lorne Michaels, Paramount Pictures and The Weinstein Company. Her film credits include Get Over It (2001), Mean Girls (2004), Hot Rod (2007), Baby Mama (2008), and Masterminds (2016).

Messick made a brief foray into television production with the short-lived NBC sitcom Bad Judge (2014–15). She was married to Producer Kevin Messick, with whom she had two children.

Messick died in Los Angeles by suicide on February 7, 2018, one day after the third anniversary of her mother's death. The method has not been reported.

The media and public attention had a negative impact on Messick. The publicity led to online cyberbullying after "activists on Twitter named, shamed and identified how Messick could be reached, with one even revealing Messick’s purposefully obscured LinkedIn profile (she did not use her full name) and encouraging others to reach out to her and let her know what they really thought of her." A Messick family statement had it that she had suffered from bipolar disorder and that a "manic episode" in 2013 had affected her career. The statement also said: "Seeing her name in headlines again and again, as part of one person's attempt to gain more attention for her personal cause, along with Harvey's desperate attempt to vindicate himself, was devastating for her. It broke Jill."