Rainsford was born in Huddersfield in Yorkshire, and later attended Repton School in Derbyshire, where he first became interested in drama. He became involved with student theatre whilst reading for a Bachelor of Arts degree in drama at the University of Manchester.
In June 2003, he joined the National Youth Theatre, and around the same time, he won a Goldman-Sachs Scholarship to the London Academy of Music and Dramatic Art. Upon graduating in 2006, his first professional engagement was as D'Artagnan in the Bristol Old Vic's swash-buckling Christmas entertainment of "The Three Musketeers".
From 2008 to 2011, Rainsford appeared to a wider audience as the disturbed son of policeman, Trevor Eve, in the television series, Waking the Dead.
In 2012, he appeared as Alexander Forbes-Blackwell in the series 26 episode of Casualty, "Ricochet: How to Save a Life". The following year, Rainsford auditioned for a regular role on Casualty as Ethan Hardy. He was offered the role, and began filming that September, with his first on-screen appearance in January 2014.
|2009||National Theater Live||Bertram|
|2011||Secret Diary-Call Girl||Tim|
|2008-11||Waking the Dead||Luke Boyd|
|2011||Law & Order UK||Jimmy Burton|
|2012-13||Call the Midwife||Jimmy|
|2016, 2018||Holby City||Ethan Hardy|