Harold Slater was born in Blackburn, Lancashire, England in 1882. By 1910 he was working as an architect in Blackburn in partnership with Harold Cooper (1877-1975). In 1910 they were placed Equal Second in a competition to design the Central Library in Blackpool. Their designs are illustrated in British Competitions in Architecture vol. 3, no. 26, November 1909 (pp. 46-47).
The practice had offices at 13 Richmond Terrace, Blackburn, and at 57-60, Central Chambers, Saskatoon, Saskatchewan, Canada. The partnership was dissolved in 1914.
The fact that the practice had an office in Saskatoon suggests that Slater may have emigrated to Canada as Cooper continued to practice in Blackburn. If he did, he had returned by England by the late 1930s as his address was given as Northroyd, Clifton Drive, South Ansdell, Lancashire in 1937, He died on 25 November 1937