Hugh Green, of Bridge of Allan, Stirlingshire, died 41 years ago, but is little known despite producing a number of exceptional watercolours.
He attended Glasgow School of Art, where he and Mackintosh became friends. But their careers diverged in 1913 when Mr Green's father died, leaving him to provide for his mother and seven younger siblings at the age of 21.
The first big exhibition of Green's work in more than 40 years will be opened on August 28 by Culture Secretary Fiona Hyslop at the Smith Museum and Art Gallery in Stirling. It is hoped the display of more than 60 watercolours plus designs and photographs will win Mr Green a place in art history.
Elspeth King, the gallery's director, said: "Hugh Green is Scotland's forgotten artist. If circumstances had been otherwise he would be well known. Though he was an exquisite artist, his works were hidden away in private collections and his story was untold."