Lewis began his single seater career by competing in the 2001 British Formula Renault Winter Series, at the age of 16. And things did not get off to the greatest of starts as he completed only three laps before crashing in his first test for Manor Motorsport. However, John Booth, who was in charge of the Manor Team, praised Lewis’ response saying “The boys put the car together again, and he went back out and went very quickly. It hadn’t fazed him in the slightest.” In the winter series he finished 5th over the four races the championship was contested but didn’t make the podium in any of them. However, his endeavours earned him a seat in the main series for 2002 with Manor. In his first race he came 3rd at Brands Hatch and took his first single seater race win at Thruxton in the 5th race of the championship. Later in the season he took back to back wins at another round in Brands hatch and Donnington, and finished the championship in third. In the same year he competed in the final four rounds of the Eurocup Formula Renault series, where he took a win at Donnington and two other 2nd places to finish 5th in the championship despite only competing in four of the nine races.
Hamilton again found himself in British Formula Renault for 2003. Whilst he failed to win any of the first four races, he won the next ten out of eleven with only a retirement at Donnington preventing him from stringing together eleven consecutive wins. With the championship wrapped up well ahead of the final round at Oulton Park, he decided to miss the weekend to participate in British F3 for the weekend instead. Where he was met with less success and finished neither race, and after the second race he was taken to hospital after crashing out. This was one of many ventures Hamilton took into other series in 2003, such as a weekend in German Formula Renault and Formula Renault Masters where he took a podium. He also competed in the Macau Grand Prix and the Korean Super Prix in which he started from pole.
In 2004, Lewis graduated to Euro F3 where he had a solid debut season, taking one win at the Norisring and five podiums across the twenty race season to finish 5th in his first year. He also participated the Masters of Formula 3 race where he finished 7th and he competed again at Macau. He also competed at the Bahrain Super Prix which he won. In the same year Hamilton was also given his first test for Mclaren. In 2005, Lewis was once again in Euro F3, but with ASM instead of Manor Motorsport making it his first season not racing for manor. This time dominated the season, winning 15 of the 20 races in the season to win the championship by almost doubling the amount of points teammate Adrian Sutil scored, who finished the season 2nd in the championship. In that years Masters of F3 race he took the Grand Chelem, winning the race from Adrian Sutil and Lucas Di Grassi. His mega season in F3 earned him a seat with ART Grand Prix for the 2006 GP2 season.