Complex sentence: In 1963, at the age of thirty, McAllister, who then resided in Meridian, won a special election in Lauderdale County to fill a vacancy in the Mississippi House, the first member of his party to serve in the legislature in the 20th century.
Split into simpler sentences: In 1963, at the age of thirty, McAllister, who then resided in Meridian, won a special election in Lauderdale County to fill a vacancy in the Mississippi House. He hence became the first member of his party to serve in the legislature in the 20th century.