He was born in Crickhowell, Brecknockshire, the younger son of the late Sir Harry Llewellyn, 3rd Bt, (d.1999) an Olympic Games Olympic gold-medalist in show jumping, and his wife, the Hon. Christine Saumarez (d.1998). He succeeded his elder brother, Dai to the baronetcy in January 2009. Llewellyn was educated at Hawtreys School and Shrewsbury School.

Starting in 1973, Llewellyn, then 25, began an affair with Princess Margaret, who was 43. The highly publicized eight year relationship was a factor in the dissolution of the Princess's stormy marriage to Antony Armstrong-Jones, 1st Earl of Snowdon.

Sir Roddy Llewellyn is now married to the former Tatiana Soskin, a daughter of the film producer Paul Soskin, and has three daughters, Alexandra, Natasha, and Rosie.

In his twenties Roddy studied horticulture at Merrist Wood Agricultural College in Surrey. Today Llewellyn runs his own Garden Design company - Roddy Llewellyn's Garden Design.

Llewellyn has given lectures at the following shows and venues: Retirement Show (2008), Dobbies Garden Centre "Inspirational Ideas for the Garden", Hereford Cathedral "Interesting Ideas for the Garden", Malvern Spring Gardening Show "Who are better Gardeners - Men or Women?", Cotswold Wildlife Park (series) "An Evening with the Gardening Experts 2005-2006", Cholmondeley Castle "Amusing Ideas for the Garden" and has travelled aboard the Crystal Symphony on a Garden Design Theme Cruise giving "Themed Lectures". He has also made an appearance on Gardeners' World (2007) and is a patron at Southport Flower Show (2006).

Llewellyn has had a accomplished journalistic career. His books include Town Gardens (1981), Beautiful Backyards (1986),Water Gardens: The Connoisseur's Choice (1987), Elegance & Eccentricity (1989), Growing Gifts (1991), Grow It Yourself. Gardening with a Physical Disability (1993), Roddy Llewellyn's Gardening Year (1997).

Llewellyn has written gardening articles for Heritage Homes (2006), Country Life (2005/2006), Country Illustrated (2004/2006), Oxfordshire Limited Edition (2006), The Lady (2005), Country Illustrated (2005), Eden Project Friends (2004), Sunday Times (2004) and columns for the 'Daily Star', from 1981 - 1985, and 'The Mail on Sunday' from 1987 - 1999. He was also the presenter of The Home Show (1990) and The Gardening Roadshow (1992 - 1993) for Thames Television amongst several others.

In the late '70s, he recorded an album called Roddy, which consisted of reworkings of standards.