Hereditary surnames were introduced to Scotland by the Normans during the 12^th century, though the use of them was not readily accepted until many years later.  Malcolm Ceannmor (1057-1093) directed his subjects to adopt surnames of their territorial possessions and thru created the first earls of Scotland. 
The Scottish surname Ralston is of local origin.  Local surnames were derived from the place where a person once lived, or a prominent local geographical feature, perhaps a hill identified one person from another who lived near a bridge, river or lake. Many of these names were preceded by a proposition such as 'de', 'at', 'by' or 'in'. These tended to lapse in later years. In this case the name is derived from the barony of Ralston near Paisley, Renfrewshire. Therefore the original bearer of this surname would have come from this place. 
Prior to the advent of surnames around the 11^th century, a person had the need for only one name, their given name. However, due to population growth and increased migration as a result of trade, economic necessity and warfare, a more precise method of identification was needed. Surnames solved this problem. Most surnames evolved from a person's occupation, where they lived, names of family members or a nickname. 
The first record of a member of this family is Nicholas de Ralstoun who noted the donation of Fulton by Sir Anthony  Lombard to the monks of Paisley in 1276 AD. Other early references include Thomas de Raulfestone of Lanarkshire, who rendered homage in 1296, and John Raleston or Raliston was an arbiter between the Burgess of Renfrew and the Abbot of Paisley during a dispote in 1488. Later records show Hugh (Hew) de Ralston of Ralston was killed in 1547 at the Battle of Pinkie and a member of this family sold the estate of Ralston to the Earl of Dundonald. 

Blazon of arms:  Argent on a bend azure three acorns in seed or. 
Translation:                    Argent represents the colours silver or white and indicates
                                      harmony and sincerity. 
Crest:                       A falcon looking to the sinister (left) proper. 
Motto:                     Fide et marte. 
Translation:                  With fidelity and bravery 
Origin:                     Scotland

From the Family Surname History Library in Edinburgh, Scotland at Whigmaleeries, Royal Mile.