Hilary was born to Wilbert and Margaret Awdry on the 12th of April 1946 and was their third child after Christopher and Veronica. She was named after the university term during which she made her entrance. Hilary's birth coincided with the month in which her father was both paid for writing and being asked to trim down Thomas the Tank Engine, which made the work inconveniently timed. As a child, she and her siblings acted as their father’s tough editors, not afraid to tell him to return to his desk and start again when he tried new material for the Railway Series.
She appeared in the Thomas the Tank Engine Man Documentary in 1995, during which she read her lecture 'Growing Up with Thomas the Tank Engine'. She later appeared alongside Christopher on BBC News and local BBC radio stations in 2010 to celebrate 65 years of Thomas & Friends.
In Autumn 2011, Hilary sent a letter to The Daily Telegraph criticising HiT Entertainment's decision to rebrand Christmas in the television series as "the winter holidays" as of eighth season. Her letter stated: "I am dismayed at 'Thomas the Tank Engine' having to call a Christmas tree a 'winter holiday tree'. It is not what my father wrote. Those who now write his stories should not take Christ out of Christmas." Because of this, the UK release of Merry Winter Wish was delayed for 2011 and many of the episodes related to Christmas from the fourteenth and fifteenth seasons were redubbed on the UK DVDs, but the episodes in the US and on Channel 5's Milkshake television broadcasts in the UK remained unchanged. However, this issue was fixed altogether in both regions starting with the seventeenth season.
Hilary was a lay minister at St Michael's and All Angels Church in Highworth, Wiltshire, England, following in her father's and grandfather's footsteps. She began training in 1987 and despite the passing of her mother Margaret in 1989 and the time she had to spend looking after her father, she completed her course and was licensed in 1990. From then on, she continued to spread the word of God right up until her death on November 25th, 2013. She was 67 and had been battling cancer for a decade. Her funeral was held the following week on December 2nd at St Michael's and All Angels Church. About 300 people turned out to pay their respects to her. She was survived by her husband Alf, her three children and six grandchildren (Emily, Rebecca, Ruth, Jack, Sam and Caitlin).
- Before she died, Hilary became a subject in a selection of portraits by the artist Dr. Susan Carr, which became part of her "Paint Me This Way" exhibition at the Swindon Museum and Art Gallery in 2017. One portrait depicted her in Wilbert Awdry's house with a model of Thomas; another depicted her as a girl beside various other pictures of different points in her life, including family photos and the C. Reginald Dalby painting of her with the rest of the Awdry family and Percy in Troublesome Engines.