《朗文》imagine

imagine somebody/something to be/do something 

I had imagined her to be older than that.

所以你的句子中的than从句没有问题的。

Marry was more thoughtful than I had imagined her to be.

1.为什么这里是要用不定式? 并没有imagine sb to do 这个结构阿,只有imagine sb doing吧?

2.这个比较句如果用imagined she was ,意思不一样嘛?

   【答】 imagine sb/sth to be sth I think they imagine the company to be bigger than it is. 我认为他们想象中的该公司比实际的要大。（摘自《麦克米伦高阶英汉双解词典》）

     I imagined him (to be) a fat man. 我把他想成胖子。(=I imagined (that) he was a fat man.)（摘自《英汉多功能词典》新编升级版p.838）

     所以，你的句子是正确的。这个比较句如果用imagined she was ,意思也一样。即：

     Marry was more thoughtful than I had imagined her to be.

    = Marry was more thoughtful than I had imagined （that）she was .