Son of the two-time rally champion, Carlos Sainz, Sainz Jr. began karting at the age of 7. He began racing karts in 2005 and – in 2006 – he won the Cadet Category of Madrid Championship. In 2008, he won the Asia-Pacific KF3 title and was runner-up in the Spanish Series. Winner of the Junior Monaco Kart Cup in 2009, Carlos was runner-up in the European KF3 Championship, also.
He became part of the Red Bull Junior Team in 2010 and won the BMW Scholarship. He competed in the Formula BMW European Championship and 3 races of the Formula BMW Asia- Pacific Series. He won the Rookie Cup Formula BMW European Championship that same year. In 2011 he continued to race in European Championships; competing in both the Northern European Cup (NEC) and the Eurocup Formula Renault 2.0.
During 2012, Sainz raced in the British and European Formula Three. He won four races in the British series and was sixth overall. In the European series, he got 2 podiums and finished in 9th overall. In 2013, he competed in GP3 and finished on the podium twice.
In 2014, Sainz competed in Formula Renault 3.5 World Championship. He won the title with seven victories, the most in a single season by any driver. In November, it was announced that he would race alongside Max Verstappen in Toro Rosso in 2015.