Elemental (liquid metal) mercury isn't especially toxic as it's not absorbed well by the body. Mercury salts, vapour and organomercury compounds (like methylmercury) are far, far more toxic.
However as others have said, even then it's a long, drawn out process over many months and with some really horrific and brutal symptoms before it is fatal.
There's a very famous case of a leading researcher who accidentally poisoned herself with a few drops of dimethylmercury:
en.m.wikipedia.org
She suffered horrendously over nearly a year before passing. It's not a good way out.