William Daniel McLennan was born in Glen Street, Paisley on 12 April 1872.
He was educated first at Mr Watson's Public School in Stevenson Street, but in 1880 he was enrolled at the John Neilson Institute.
Very little is known about where he trained, but there is strong evidence that he was in the practice of William Henry Lynn in Belfast sometime between 1890 and 1895. Lynn had previously designed Paisley Town Hall.
In 1895 he was amongst those invited to compete for Paisley Technical School and achieved some distinction by beating several leading practices to third place in Rowand Anderson's report. This encouraged him to set up practice in Paisley late in 1896 or early in 1897.