This strikes me as odd, how did Palpatine obtain Mas Amedda's complete loyalty even after revealing some, if not all, of his secrets, did Mas Amedda hate the Jedi and/or the Republic? What exactly moved him to aid in Palpatine's plans to create a fake war that would end up in the death of all Jedi?
There's very little to go on, but I did find this passage about Mas Amedda's internal thought process from 'Aftermath: Life Debt' to be very telling.
"It is lonely being the head of a dying Empire.
He is a figurehead. Or worse than one. They don’t even trot him out for appearances. His office and his chambers make up his prison. It’s here he mostly stays. Taking in his meals. Watching the HoloNet. Thinking about his future, or rather, the lack of his future.
It wasn’t supposed to be like this.
Palpatine was supposed to remain. The Emperor was as certain a fixture in the galaxy as the Core itself. As fundamental as the Imperial Palace. Timeless and immortal."
My read on it is that Mas is a follower, not a leader and Palpatine promised him a place of power and prestige in his Empire. This is especially evident elsewhere in the book where it's made clear he has no idea what to do without Palpatine.
It probably started small. A secret here, a favour there. At some point he saw Sidious's true face and for whatever reason, decided it was better (and probably healthier) to stand by him than against him.