The Briton, 28, won a hard-fought points decision on Saturday to stretch his unbeaten record to 25 fights.
Saunders dominated an uninspiring contest at the Copper Box Arena, with all three judges giving him the decision - 117-111, 115-114, 117-112.
It was his second successful defence of the one middleweight world title not held by Gennady Golovkin.
Saunders had hoped to fight the winner of Saturday's Las Vegas bout between the Kazakh and Saul 'Canelo' Alvarez in a unification fight, but that much-hyped contest ended in a controversial draw.
Promoter Frank Warren had previously described a fight with Golovkin in December as a "done deal", but said that "worst-case scenario, Bill will fight again against someone else in December".
Saunders echoed that sentiment, and described his performance as a "big improvement" on his last defence - a defeat of Artur Akavov 10 months ago.
"There was stuff I wanted to try but if someone is running around on the back foot, it's hard. Every time I wanted to meet him in the middle he would run, but I got the win and that's the main thing."
After the fight, Saunders' son Stevie apologised to Monroe for punching him at Friday's weigh-in.