Theo Lewis began his racing career in 1923 as an apprentice with Richard Bradfield at Flemington. His greatest success as a jockey was his winning of two Williamstown Cups on Second Wind, whom he rode into second place behind Phar Lap in the 1930 Melbourne Cup.

Lewis switched to training in the mid 1930s and won his first race with Lord Carrington in December, 1936. The following month Lord Carrington won the Geelong Cup, the first of many Cup victories for which Lewis became famous. These included the Australian Cup (twice), Brisbane Cup (twice), Sandown Cup, Caulfield Cup, King's Cup (twice), Port Adelaide Cup (three times), Launceston Cup and a host of provincial cups. Lewis also captured numerous feature handicap and weight-for-age events, among which were three Toorak Handicaps, the Newmarket Handicap, the Standish Handicap and four Bagot Handicaps, the Caulfield Guineas, two VRC St Legers and the Underwood Stakes.

His best horse was St Fairy which he trained for owner Ossie Porter, with whom he had a long association. As a trainer of jumpers he also had success, winning the Australian Hurdle with Red Pampas in 1958.

Lewis won the Victorian trainers' premiership in 1952-53, and shared it with Fred Hoysted in 1955-56. He died of a heart attack in 1962 at the age of 52, one day after being elected President of the Victorian Trainers' Association.

Lewis was inducted into the Australian Racing Hall of Fame in 2012.