The Supreme Chancellor is both the Head of State and Head of Government for the Galactic Republic. A Chancellor is elected into office by the Galactic Senate to serve a set number of terms. Oftentimes a Chancellor is voted into office from the current members of the Senate, and a new Senator is concurrently voted into place for that planet during the same meeting. They can be removed from office by loss of an election, by serving the maximum number of terms, or through a vote of no confidence in the Senate.
The office was held by Supreme Chancellor Janarus up until the government of Corellia seceded to the Empire. A bounty hunter, whom Janaras had placed at the top of the Republic's Most Wanted list, helped discourage any hopes of Corellia rejoining the Republic. Soon after, Janaras was tracked and removed from office by the same hunter, at which point Leontyne Saresh, the governor of Taris and Janarus' political rival, assumed the office.
- For information on this subject beyond The Old Republic, see: Supreme Chancellor on Wookieepedia