The A753 was originally numbered as the short lived B714), but upgraded to Class I status in 1925 or 26. It ran eastwards from the A70 at Carnwath in Lanarkshire, passing through Newbigging and Elsrickle before crossing the A702 and then entering Peeblesshire. The route ended on the A72 at Kirkdean near Blyth Bridge. The A753 became the eastern part of the A721 in 1934 when that route was extended towards Peebles.
An official document from 16/05/1934 details the following changes: The whole of this route to be renumbered as A721