The 43-year-old presenter - who has children Austin, 16, and Arion, 12, with ex-husband Kyle Boyd - has been with the network for 12 years, and over the last 12 months has ''inherited a lot more responsibility and work hours'', so was pleased when executives told her they wanted to extend her contract as it was coming to an end.
But at the same time, Cat was stunned to learn Jason was ''making double [her] salary and has been for several years.''
She added to People magazine: ''That was really hard to swallow, but you know information is supposed to be power and when my team began negotiations knowing what we knew, that was the barometer in which I expected to be paid, based on the law and based on what I know to be fair. And what I believe in my heart of hearts is reasonable.''
However, though Catt asked for equal pay to her co-host and friend, she insisted the offer the network returned with was ''not even remotely close'' to his salary.
After much soul searching, the 'Daily Pop' presenter decided to walk away and not sign a new contract.
She said: ''It's almost insulting because you know you work really hard. I'm a single mom of two kids. I've given my all to this network.
''I've sacrificed time away from my family and I have dedicated my entire career to this network. And when you learn something like that, it makes you feel very small and underappreciated and undervalued. It's heartbreaking...
''If I stay and do the easy thing, I don't serve myself and I don't serve every other female in the world.
''It's like I now feel inspired and empowered by these women before me who refused to be silent.''
A spokesperson for the network insisted their staff were paid ''fairly and appropriately''.
They said in a statement: ''E! compensates employees fairly and appropriately based on their roles, regardless of gender. We appreciate Catt Sadler's many contributions at E! News and wish her all the best following her decision to leave the network.''