Sadler never names Jason Kennedy in her post, but the pair co-hosted Daily Pop.
In negotiations with the network, she said she and her team "asked for what I know I deserve and were denied repeatedly".
She described her decision to leave in the post: "Know your worth. I have two decades experience in broadcasting and started at the network the very same year as my close friend and colleague that I adore. I so lovingly refer to him as my 'TV husband' and I mean it.
"But how can I operate with integrity and stay on at E if they're not willing to pay me the same as him? Or at least come close? How can I accept an offer that shows they do not value my contributions and paralleled dedication all these years?
"How can I not echo the actions of my heroes and stand for what is right no matter what the cost? How can I remain silent when my rights under the law have been violated?"
A spokesperson for E! did not return a message seeking comment.
Sadler's admission put a fresh and famous face on an issue that has vexed this country since women started entering the workforce in droves at the end of World War II.
According to the US Census Bureau, women working full time in 2016 made 81 cents for each dollar made by a man.
As the New York Times reported, the gap is larger among college graduates, and "far larger in business, finance and legal careers than in science and technology jobs." The gap increases with age - and when women marry and begin having children.
Under President Donald Trump, the White House's gender pay gap more than tripled, as The Washington Post's Christopher Ingraham wrote in July: The median female White House employee is drawing a salary of US$72,650 in 2017, compared to the median male salary of US$115,000.
And in August, the Trump administration halted an Obama-era policy that would have required large companies to report pay data broken down by race and gender, a move that incensed organisations that have lobbied for better policies around equal pay.
Some of the pay gap's most vocal critics have been women like Sadler, who already have a megaphone of sorts.
In 2011, for example, Morning Joe co-host Mika Brzezinski revealed that Joe Scarborough, the MSNBC show's namesake, made 14 times her salary.
"It was my fault, ultimately, that I was in that position, because I accepted a deal that wasn't right for me," she said, according to Jezebel.
She said it ultimately "took four tries" with MSNBC's Phil Griffin "to get it right for myself".
In 2015, Jennifer Lawrence, the Oscar winner and Hunger Games megastar, penned a Facebook post called "Why Do I Make Less Than My Male Co-Stars?"
"I don't think I've ever worked for a man in charge who spent time contemplating what angle he should use to have his voice heard," she wrote. "It's just heard.
"Jeremy Renner, Christian Bale, and Bradley Cooper all fought and succeeded in negotiating powerful deals for themselves. If anything, I'm sure they were commended for being fierce and tactical, while I was busy worrying about coming across as a brat and not getting my fair share."
In her blog post, Sadler said she wanted to set a good example for her two children.
She said she was also inspired by women who had come forward with stories of sexual assault and sexual harassment, launching the #MeToo movement that continues to resonate.
"Countless brave women have come forward this year to speak their truth," Sadler wrote. "Females refuse to remain silent on issues that matter most because without our voices, how will we invoke lasting change? How can we make it better for the next generation of girls if we do not stand for what is fair and just today?"
