Even though we don't really meet Sam Westing as "Sam Westing" during the course of the book, we do meet several versions of him in the disguises he uses. His four identities correspond to the four cardinal directions: Barney Northrup, Julian Eastman, Sandy McSouthers, and Sam Westing. The real Sam Westing—whatever that means—is, and was, a tremendously successful businessman.
For much of the book we think he was murdered by one of the people playing the Westing game—at least, that's what he wants us to believe.
What we know of him comes from the language he used to write the will, the obituary printed about him in the paper, and the nibbles and bits of information the characters collect from one another. We know he was married, but his relationship fell apart when his only daughter died. He's the son of immigrants and lived out the American Dream, rising from nothing to become the head of a $200 million dollar corporation. He became shrouded in mystery after being severely injured in a car crash. He loves disguises, dress-up, and play-acting; he also loves games of strategy like chess, and he hardly ever loses. He's tremendously patriotic and, as we can see from the game he designed for his heirs to play, he's very, very clever.
Westing's also a philanthropist: he funded the education of his servants' daughter, J.J. Ford, and he designs all the pairs in the Westing game so that each person will end up with someone who can best help him/her. It seems that, as Julian Eastman, he also pays for Turtle's education. We see this legacy of philanthropy passed down to the people he helped, as Judge Ford pays for Chris's education and Turtle contributes to Alice's.