He was the son of a prosperous builder, engineer and railway-contractor, Samuel Morton Peto, of Somerleyton Hall in Lowestoft, Suffolk, and of Sarah Ainsworth (née Kelsll), his father's second wife. Harold had four step-brothers and -sisters and ten brothers and sisters.
1876 Peto entered into a partnership with Streatham resident Ernest George (1839 – 1922) an established architect and artist.
Together they designed a number of buildings in Streatham including Ernest George's "Redroofs" overlooking Streatham Common and Hambly Mansions