Crichlow's father was Frank Gilbert Crichlow, a community activist originally from the island of Trinidad in the Caribbean, who once ran the Mangrove restaurant in Notting Hill which was frequented by stars such as Jimi Hendrix and Diana Ross. He died on 15 September 2010. Her mother, Lucy Addington, is a white Englishwoman. She has two sisters, Amandla and Francesca, and a brother, Knowlton. Her younger sister, Amandla Crichlow, is also an actress and has appeared in Prime Suspect: The Final Act and The Bill.
Crichlow came to public prominence starring as Maria "Sugar" Sweet in Channel 4's adaptation of the Julie Burchill novel Sugar Rush, which won the 2006 International Emmy Award in the 'Children And Young People' category.
She then appeared in "Gridlock", a 2007 episode of Doctor Who, and BBC One's feature-length drama Kiss of Death in 2008, starring alongside Danny Dyer. In March 2010, Lenora returned to the world of Doctor Who in the audio release of Doctor Who: The Architects of History, playing a new character "Rachel."
In November 2008, the BBC announced she had been cast as Annie, a ghost in a flat shared with a vampire and a werewolf, in the BBC Three series Being Human, replacing Andrea Riseborough who had played the character in the pilot. She continued to play the role of Annie in the programme until the last episode of the fourth season, in March 2012.
Since January 2010, Crichlow has appeared in Material Girl on BBC One as Ali Redcliffe, a fashion designer. She in June 2010 also played the role of Ashley in a BBC Three pilot, named Dappers.
In 2011, Crichlow played the character of Police Sergeant Lily Thomson in the BBC drama Death in Paradise. In 2012 Crichlow appeared in an episode of Inspector George Gently and also played the central role of Shania, a streetwise 200m runner, in the Olympic themed film Fast Girls. On 26 December she starred as Laura Stanton in the ITV1 Drama Doors Open, an adaptation of the Ian Rankin novel of the same name.