Weinstein, once one of Hollywood's most powerful producers, is facing up to 29 years in prison after a New York City jury found him guilty of attacking two women.
The disgraced movie mogul was found guilty of rape in the third degree but cleared of the most serious charges of predatory sexual assault.
Ms McGowan was one of the earliest - and most prominent - of Weinstein's accusers.
She said: "I wanted to come and be the voice for those who will never get the chance to stand.
"You have to be able to judge for yourself what you're going to be able to handle because I can tell you it will be the fight of your life."
Asked if the potential 29 years Weinstein could face in prison is "enough", Ms Mcgowan said: "Yes, for me, personally".
The 46-year-old added: "I would just rather he cease to exist. I think the world would be better off, but I'm happy there is one less serial predator on the streets".
Reflecting on her journey since accusing Weinstein in 2017, Ms McGowan said: "The young girl that I was is giving a high five to the woman I am today".
New York County District Attorney describes Weinstein as a "vicious, serial sexual predator"
Weinstein's New York trial was the first criminal case to arise from a barrage of allegations against the movie mogul.
More than 90 women came forward, including actresses Gwyneth Paltrow, Salma Hayek and Uma Thurman.
The 67-year-old was found guilty of a criminal sex act for assaulting production assistant Mimi Haleyi at his apartment in 2006 and third-degree rape of a woman in 2013.
The jury of seven men and five women took five days to find him guilty.
Following the verdicts, New York County District Attorney, Cyrus Vance, described Weinstein as a "vicious, serial sexual predator".
The case against the once-feared producer was essentially built on three allegations: that he raped an aspiring actress in a New York City hotel room in 2013, that he forcibly performed oral sex on another woman at his apartment in 2006, and that he raped and forcibly performed oral sex on another actress in her apartment in the mid-1990s.
Following his conviction, a number of public figures have praised the women who testified against Weinstein.
Double Jeopardy star Ashley Judd, one of Weinstein's earliest accusers, tweeted: "For the women who testified in this case, and walked through traumatic hell, you did a public service to girls and women everywhere, thank you."
Actress Rosanna Arquette, another of Weinstein's high-profile accusers, tweeted: "Gratitude to the brave women who've testified and to the jury for seeing through the dirty tactics of the defence."
She added: "We will change the laws in the future so that rape victims are heard and not discredited, and so that it's easier for people to report their rapes."
Former Sopranos star Annabella Sciorra gave evidence in court that Weinstein had raped her in her New York City apartment in the mid-1990s.
Weinstein was acquitted on the two counts of predatory sexual assault that hinged on Ms Sciorra's evidence.
In a statement following the verdict, she said: "My testimony was painful but necessary. I spoke for myself and with the strength of the 80-plus victims of Harvey Weinstein in my heart.
"While we hope for continued righteous outcomes that bring absolute justice, we can never regret breaking the silence. "For in speaking truth to power we pave the way for a more just culture, free of the scourge of violence against women."
While Weinstein did not testify, his lawyers contended any sexual contact was consensual and that his accusers slept with him to advance their careers.
His defence seized on the fact two of the women central to the case stayed in contact with Weinstein and were flirty in email exchanges, and had sex with him, long after he allegedly attacked them.
Weinstein helped bring to the screen such Oscar winners as Good Will Hunting, Pulp Fiction, The King’s Speech and Shakespeare in Love and nurtured the careers of celebrated filmmakers like Quentin Tarantino and Kevin Smith.
Weinstein will remain in custody until his sentencing on 11 March, where the New York District Attorney confirmed he will serve no less than five years.
Weinstein is also facing charges in Los Angeles.