Recent evidence shows that Plympton became a borough before 1155. A number of charters granted over a period of 600 years exist. By 1602 Plympton was entitled to elect a mayor and other officers of the borough and to hold annual fairs and a weekly market. For 537 years from 1295, Plympton returned two members to Parliament. The representatives were drawn from families of local standing - such as the Strodes, Heles, Drakes and Trebys and others such as Sir Christopher Wren who was elected in 1685 and Viscount Castelreagh, Minister of War and Foreign Secretary during the Napoleonic Wars, who was a member for Plympton in 1807. Prior to the Reform Act of 1832 the voters of Plympton received many favours from local patrons such as the Trebys, and later, the Earls of Mount Edgcumbe. With the introduction of the Reform Act, Plympton lost its right to elect two M.Ps. and became absorbed into a county constituency. Consequently, the Earl of Mount Edgcumbe lost interest and withdrew his financial support. After 700 years, Plympton could no longer afford to be a borough and the last mayor was elected in 1859. Plympton St. Maurice parish council was established in 1894, but in 1967, by Act of Parliament, administration passed to the hands of Plymouth. It was in 1968 that Plymouth City Council designated Plympton St. Maurice as a Conservation Area, and the local Civic Association dates from that time. Plympton's former civic status reflected its economic importance.
A tidal creek extended from the Plym estuary close to Plympton Priory which adjoined Plympton St. Mary church. It is unlikely that boats of any size penetrated far upstream, although excavations at the junction of Underwood and Dark Street Lane have revealed what might be part of a stone quay and oak mooring posts. The fact that a strip of land belonging to the Borough extended westwards along Underwood to the Plym estuary by Saltram, suggests that this is where most ships moored. In the twelfth century, ships carried slate from Plympton to Southampton, and an old document describes how in 1272 ships were bringing hemp from Bridport to be made into rope yarn. In Elizabethan times the Mayor of Plympton, John Martin, traded with France and Spain in his vessel "John of Plympton", taking out wool and tin and bringing back wine. The industries of mediaeval Plympton included rope making, coopering, brewing, cider making, tanning, wool combing and hat making.