Kendall grew up in the village of Abbots Langley in Hertfordshire and attended Watford Grammar School for Girls. She then went up to Queens' College, Cambridge, graduating with a first in history in 1993.
Unsuccessful in an attempt to become the Labour candidate in Chesterfield for the 2001 general election, she has worked as Director of the Maternity Alliance charity, as a researcher for the King's Fund, as an associate director for health, social care and children’s early years at the IPPR and has been aspecial adviser to two cabinet ministers, Patricia Hewitt and Harriet Harman.
On 26 May 2010 Kendall posted a picture of the previous day's state opening of parliament on the microblogging website Twitter, in contravention of parliamentary rules. She apologised, saying that she was unaware of the regulations.
In 2011, she was made the Shadow Minister for Care and Older People by Ed Miliband, entitling her to attend meetings of the Shadow Cabinet, though not as a full member. On 12 February 2013 she gave a speech on health care reform at the National Health Service. On 18 October of the same year she promised to take action against 40 youths who were targeting a single family whose car they had burned twice.