Little-known social coding start-up GitHub Inc. has raised $100 million in its first round of funding, in a sign of how big investment bets are continuing in Silicon Valley.

The round also marks the biggest check written to date by venture-capital firm Andreessen Horowitz, which is leading GitHub's financing and putting in more than 99% of the money. The funding values four-year-old GitHub at $750 million, said a person briefed about the deal.

"It's...