Used to emphasize an even stronger or greater case than the one just mentioned — 'much less' / 'let alone' / 'still more'.
子どもでもできる問題だ、まして大人ならすぐ解けるだろう。
It's a problem even a child can solve; all the more so an adult would solve it quickly.
彼女は英語が堪能だ。まして会議での発表は安心して任せられる。
She's fluent in English. What’s more, you can confidently leave the conference presentation to her.
寒さが厳しい。まして標高の高い場所ではなおさらだ。
The cold is severe; it's all the worse at higher elevations.
Used with a negative verb to mean 'even less' or 'let alone' (i.e., something is even less likely/possible).
彼は走るのも遅い。ましてマラソンで勝てるはずがない。
He's slow even at running; let alone could he win a marathon.
忙しくて手が回らない。まして新しいプロジェクトを引き受ける余裕はない。
I'm too busy to manage; I certainly have no capacity to take on a new project.