Robert came from the branch of the Harrington family which held Little Panton in Lincolnshire, although as a descendant of a younger son of John Harrington (d.1326) of Panton he could lay claim neither to the Lincolnshire estates nor to those in Northamptonshire and Rutland which all passed in 1376 to three female coheirs. However, Robert’s grandfather, Thomas, held the manor of Glaston in Rutland, which came into the possession of the MP’s father, John, before 1353. John’s severe financial difficulties, exacerbated by costly litigation arising from the claims to Glaston made by his more prominent kinsman, Sir John Harrington, led him to sell a life interest in the manor to William Wade of Stockerston and his wife Margaret, and to promise to Sir Roland Daneys the reversion. Robert Harrington’s first task was therefore to regain this part of his inheritance. Following the deaths of the Wades in 1364 and 1377, respectively, he challenged the right of Daneys’s nephew and heir, John Daneys of Tickencote, to succeed to Glaston and finally, in 1381, secured from him a formal quitclaim. Perhaps he had to buy him out.
It was not until after his first election to Parliament that Sir Robert was found employment as a royal commissioner and j.p. Little is known about those with whom he usually associated, save that on occasion he witnessed deeds for members of the Hastings family. From 1387 to 1392 he was engaged in a suit over the manor of Carlton Curlieu (Leicestershire) in which one of his fellow litigants was William Chiselden, receiver-general of the duchy of Lancaster.